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SubjectRe: [PATCH] fs/exec: require argv[0] presence in do_execveat_common()
On Wed, Jan 26, 2022 at 04:39:47AM +0000, Ariadne Conill wrote:
> The first argument to argv when used with execv family of calls is
> required to be the name of the program being executed, per POSIX.
>
> By validating this in do_execveat_common(), we can prevent execution
> of shellcode which invokes execv(2) family syscalls with argc < 1,
> a scenario which is disallowed by POSIX, thus providing a mitigation
> against CVE-2021-4034 and similar bugs in the future.
>
> The use of -EFAULT for this case is similar to other systems, such
> as FreeBSD and OpenBSD.
>
> Interestingly, Michael Kerrisk opened an issue about this in 2008,
> but there was no consensus to support fixing this issue then.
> Hopefully now that CVE-2021-4034 shows practical exploitative use
> of this bug in a shellcode, we can reconsider.
>
> Signed-off-by: Ariadne Conill <ariadne@dereferenced.org>

Yup. Agreed. For context:
https://www.qualys.com/2022/01/25/cve-2021-4034/pwnkit.txt

> ---
> fs/exec.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index 79f2c9483302..de0b832473ed 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1897,8 +1897,10 @@ static int do_execveat_common(int fd, struct filename *filename,
> }
>
> retval = count(argv, MAX_ARG_STRINGS);
> - if (retval < 0)
> + if (retval < 1) {
> + retval = -EFAULT;
> goto out_free;
> + }

There shouldn't be anything legitimate actually doing this in userspace.

-Kees

> bprm->argc = retval;
>
> retval = count(envp, MAX_ARG_STRINGS);
> --
> 2.34.1
>

--
Kees Cook

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