Re: [PATCH] exec: Force single empty string when argv is empty

[Rich Felker]

On Mon, Jan 31, 2022 at 04:09:47PM -0800, Kees Cook wrote:
> Quoting[1] Ariadne Conill:
> 
> "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[2]:
> 
>     The argument arg0 should point to a filename string that is
>     associated with the process being started by one of the exec
>     functions.
> ....
> Interestingly, Michael Kerrisk opened an issue about this in 2008[3],
> but there was no consensus to support fixing this issue then.
> Hopefully now that CVE-2021-4034 shows practical exploitative use[4]
> of this bug in a shellcode, we can reconsider.
> 
> This issue is being tracked in the KSPP issue tracker[5]."
> 
> While the initial code searches[6][7] turned up what appeared to be
> mostly corner case tests, trying to that just reject argv == NULL
> (or an immediately terminated pointer list) quickly started tripping[8]
> existing userspace programs.
> 
> The next best approach is forcing a single empty string into argv and
> adjusting argc to match. The number of programs depending on argc == 0
> seems a smaller set than those calling execve with a NULL argv.
> 
> Account for the additional stack space in bprm_stack_limits(). Inject an
> empty string when argc == 0 (and set argc = 1). Warn about the case so
> userspace has some notice about the change:
> 
>     process './argc0' launched './argc0' with NULL argv: empty string added
> 
> Additionally WARN() and reject NULL argv usage for kernel threads.
> 
> [1] https://lore.kernel.org/lkml/20220127000724.15106-1-ariadne@xxxxxxxxxxxxxxxx/
> [2] https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html
> [3] https://bugzilla.kernel.org/show_bug.cgi?id=8408
> [4] https://www.qualys.com/2022/01/25/cve-2021-4034/pwnkit.txt
> [5] https://github.com/KSPP/linux/issues/176
> [6] https://codesearch.debian.net/search?q=execve%5C+*%5C%28%5B%5E%2C%5D%2B%2C+*NULL&literal=0
> [7] https://codesearch.debian.net/search?q=execlp%3F%5Cs*%5C%28%5B%5E%2C%5D%2B%2C%5Cs*NULL&literal=0
> [8] https://lore.kernel.org/lkml/20220131144352.GE16385@xsang-OptiPlex-9020/
> 
> Reported-by: Ariadne Conill <ariadne@xxxxxxxxxxxxxxxx>
> Reported-by: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
> 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: linux-fsdevel@xxxxxxxxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  fs/exec.c | 26 +++++++++++++++++++++++++-
>  1 file changed, 25 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index 79f2c9483302..bbf3aadf7ce1 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -495,8 +495,14 @@ static int bprm_stack_limits(struct linux_binprm *bprm)
>  	 * the stack. They aren't stored until much later when we can't
>  	 * signal to the parent that the child has run out of stack space.
>  	 * Instead, calculate it here so it's possible to fail gracefully.
> +	 *
> +	 * In the case of argc = 0, make sure there is space for adding a
> +	 * empty string (which will bump argc to 1), to ensure confused
> +	 * userspace programs don't start processing from argv[1], thinking
> +	 * argc can never be 0, to keep them from walking envp by accident.
> +	 * See do_execveat_common().
>  	 */
> -	ptr_size = (bprm->argc + bprm->envc) * sizeof(void *);
> +	ptr_size = (min(bprm->argc, 1) + bprm->envc) * sizeof(void *);

>From #musl:

<mixi> kees: shouldn't the min(bprm->argc, 1) be max(...) in your patch?

I'm pretty sure without fixing that, you're introducing a giant vuln
here. I believe this is the second time a patch attempting to fix this
non-vuln has proposed adding a new vuln...

Rich