Re: [PATCH v2] fs/exec: require argv[0] presence in do_execveat_common()

[Kees Cook <keescook@chromium.org>]

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

Okay, so, the dangerous condition is userspace iterating through envp
when it thinks it's iterating argv.

Assuming it is not okay to break valgrind's test suite:
https://sources.debian.org/src/valgrind/1:3.18.1-1/none/tests/execve.c/?hl=22#L22
we cannot reject a NULL argv (test will fail), and we cannot mutate
argc=0 into argc=1 (test will enter infinite loop).

Perhaps we need to reject argv=NULL when envp!=NULL, and add a
pr_warn_once() about using a NULL argv?

I note that glibc already warns about NULL argv:
argc0.c:7:3: warning: null argument where non-null required (argument 2)
[-Wnonnull]
    7 |   execve(argv[0], NULL, envp);
      |   ^~~~~~

in the future we could expand this to only looking at argv=NULL?

-- 
Kees Cook