linux-kernel - Re: [PATCH v2] fs/exec: require argv[0] presence in do_execveat

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Date:   Wed, 26 Jan 2022 18:03:51 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Ariadne Conill <ariadne@...eferenced.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Kees Cook <keescook@...omium.org>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v2] fs/exec: require argv[0] presence in
 do_execveat_common()

On Wed, Jan 26, 2022 at 10:57:29AM -0600, Eric W. Biederman wrote:
> Matthew Wilcox <willy@...radead.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'm wondering if I misremembered and it's being done in ld.so
rather than in the kernel?  That might be the right place to put
this fix too.

> 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 like that better than NULL.