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Message-ID: <9e276534-5c97-67fc-4bae-8de11bab57ab@dereferenced.org>
Date: Wed, 26 Jan 2022 11:32:38 -0600 (CST)
From: Ariadne Conill <ariadne@...eferenced.org>
To: "Eric W. Biederman" <ebiederm@...ssion.com>
cc: Matthew Wilcox <willy@...radead.org>,
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()
Hi,
On Wed, 26 Jan 2022, 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 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
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