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SubjectRe: [PATCH v2] fs/exec: require argv[0] presence in do_execveat_common()
On Wed, Jan 26, 2022 at 11:44:47AM +0000, Ariadne Conill wrote:
> In several other operating systems, it is a hard requirement that the
> first argument to execve(2) be the name of a program, thus prohibiting
> a scenario where argc < 1. POSIX 2017 also recommends this behaviour,
> but it is not an explicit requirement[0]:
>
> The argument arg0 should point to a filename string that is
> associated with the process being started by one of the exec
> functions.
>
> To ensure that execve(2) with argc < 1 is not a useful gadget for
> shellcode to use, we can validate this in do_execveat_common() and
> fail for this scenario, effectively blocking successful exploitation
> of CVE-2021-4034 and similar bugs which depend on this gadget.
>
> The use of -EFAULT for this case is similar to other systems, such
> as FreeBSD, OpenBSD and Solaris. QNX uses -EINVAL for this case.
>
> Interestingly, Michael Kerrisk opened an issue about this in 2008[1],
> 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.
>
> [0]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html
> [1]: https://bugzilla.kernel.org/show_bug.cgi?id=8408
>
> Changes from v1:
> - Rework commit message significantly.
> - Make the argv[0] check explicit rather than hijacking the error-check
> for count().
>
> Signed-off-by: Ariadne Conill <ariadne@dereferenced.org>
> ---
> fs/exec.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index 79f2c9483302..e52c41991aab 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1899,6 +1899,10 @@ static int do_execveat_common(int fd, struct filename *filename,
> retval = count(argv, MAX_ARG_STRINGS);
> if (retval < 0)
> goto out_free;
> + if (retval == 0) {
> + retval = -EFAULT;
> + goto out_free;
> + }

I don't object to the concept, but it's a more common pattern in Linux
to do this:

retval = count(argv, MAX_ARG_STRINGS);
+ if (retval == 0)
+ retval = -EFAULT;
if (retval < 0)
goto out_free;

(aka I like my bikesheds painted in Toasty Eggshell)

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