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

[Ariadne Conill <ariadne@dereferenced.org>]

Hi,

On Wed, 26 Jan 2022, Eric W. Biederman wrote:

> Matthew Wilcox <willy@infradead.org> writes:
>
>> On Wed, Jan 26, 2022 at 11:44:47AM +0000, Ariadne Conill wrote:
>>> 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
>>
>> Having now read 8408 ... if ABI change is a concern (and I really doubt
>> it is), we could treat calling execve() with a NULL argv as if the
>> caller had passed an array of length 1 with the first element set to
>> NULL.  Just like we reopen fds 0,1,2 for suid execs if they were
>> closed.
>
> Where do we reopen fds 0,1,2 for suid execs?  I feel silly but I looked
> through the code fs/exec.c quickly and I could not see it.
>
>
> I am attracted to the notion of converting an empty argv array passed
> to the kernel into something we can safely pass to userspace.
>
> I think it would need to be having the first entry point to "" instead
> of the first entry being NULL.  That would maintain the invariant that you
> can always dereference a pointer in the argv array.

Yes, I think this is correct, because there's a lot of programs out there 
which will try to blindly read from argv[0], assuming it is present. 
Ensuring we wind up with {"", NULL} would be the way I would want to 
approach this if we go that route.

This approach would solve the problem with pkexec, but I still think there 
is some wisdom in denying with -EFAULT outright like other systems do.

Ariadne