Re: [PATCH v3] fs/exec: require argv[0] presence in do_execveat_common()

[Kees Cook]

On Thu, Jan 27, 2022 at 12:07:24AM +0000, Ariadne Conill wrote:
> In several other operating systems, it is a hard requirement that the
> second 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 tool 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 execve(2) working
> with argc < 1.
> 
> We use -EINVAL for this case, mirroring recent changes to FreeBSD and
> OpenBSD.  -EINVAL is also used by QNX for this, while Solaris uses
> -EFAULT.
> 
> In earlier versions of the patch, it was proposed that we create a
> fake argv for applications to use when argc < 1, but it was concluded
> that it would be better to just fail the execve(2) in these cases, as
> launching a process with an empty or NULL argv[0] was likely to just
> cause more problems.

Let's do it and see what breaks. :)

I do see at least tools/testing/selftests/exec/recursion-depth.c will
need a fix. And maybe testcases/kernel/syscalls/execveat/execveat.h
in LTP.

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

> 
> 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[2]
> of this bug in a shellcode, we can reconsider.
> 
> This issue is being tracked in the KSPP issue tracker[3].
> 
> There are a few[4][5] minor edge cases (primarily in test suites) that
> are caught by this, but we plan to work with the projects to fix those
> edge cases.
> 
> [0]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html
> [1]: https://bugzilla.kernel.org/show_bug.cgi?id=8408
> [2]: https://www.qualys.com/2022/01/25/cve-2021-4034/pwnkit.txt
> [3]: https://github.com/KSPP/linux/issues/176
> [4]: https://codesearch.debian.net/search?q=execve%5C+*%5C%28%5B%5E%2C%5D%2B%2C+*NULL&literal=0
> [5]: https://codesearch.debian.net/search?q=execlp%3F%5Cs*%5C%28%5B%5E%2C%5D%2B%2C%5Cs*NULL&literal=0
> 
> Changes from v2:
> - Switch to using -EINVAL as the error code for this.
> - Use pr_warn_once() to warn when an execve(2) is rejected due to NULL
>   argv.
> 
> Changes from v1:
> - Rework commit message significantly.
> - Make the argv[0] check explicit rather than hijacking the error-check
>   for count().
> 
> Reported-by: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
> To: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Christian Brauner <brauner@xxxxxxxxxx>
> Cc: Rich Felker <dalias@xxxxxxxx>
> Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx>
> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Ariadne Conill <ariadne@xxxxxxxxxxxxxxxx>
> ---
>  fs/exec.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index 79f2c9483302..982730cfe3b8 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1897,6 +1897,10 @@ static int do_execveat_common(int fd, struct filename *filename,
>  	}
>  
>  	retval = count(argv, MAX_ARG_STRINGS);
> +	if (retval == 0) {
> +		pr_warn_once("Attempted to run process '%s' with NULL argv\n", bprm->filename);
> +		retval = -EINVAL;
> +	}
>  	if (retval < 0)
>  		goto out_free;
>  	bprm->argc = retval;
> -- 
> 2.34.1
> 

-- 
Kees Cook