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From:   "Eric W. Biederman" <ebiederm@xmission.com>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Ariadne Conill <ariadne@dereferenced.org>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        Kees Cook <keescook@chromium.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>
References: <20220126114447.25776-1-ariadne@dereferenced.org>
        <YfFh6O2JS6MybamT@casper.infradead.org>
Date:   Wed, 26 Jan 2022 10:57:29 -0600
In-Reply-To: <YfFh6O2JS6MybamT@casper.infradead.org> (Matthew Wilcox's message
        of "Wed, 26 Jan 2022 14:59:52 +0000")
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Subject: Re: [PATCH v2] fs/exec: require argv[0] presence in
 do_execveat_common()
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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.

Eric