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Date:   Wed, 26 Jan 2022 14:23:59 -0600 (CST)
From:   Ariadne Conill <ariadne@dereferenced.org>
To:     Kees Cook <keescook@chromium.org>
cc:     Ariadne Conill <ariadne@dereferenced.org>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        Eric Biederman <ebiederm@xmission.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH v2] fs/exec: require argv[0] presence in
 do_execveat_common()
In-Reply-To: <202201261202.EC027EB@keescook>
Message-ID: <a8fef39-27bf-b25f-7cfe-21782a8d3132@dereferenced.org>
References: <20220126114447.25776-1-ariadne@dereferenced.org> <202201261202.EC027EB@keescook>
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Hi,

On Wed, 26 Jan 2022, Kees Cook wrote:

> On Wed, Jan 26, 2022 at 11:44:47AM +0000, Ariadne Conill wrote:
>> In several other operating systems, it is a hard requirement that the
>> first 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[0]:
>>
>>     The argument arg0 should point to a filename string that is
>>     associated with the process being started by one of the exec
>>     functions.
>>
>> To ensure that execve(2) with argc < 1 is not a useful gadget for
>> shellcode to use, we can validate this in do_execveat_common() and
>> fail for this scenario, effectively blocking successful exploitation
>> of CVE-2021-4034 and similar bugs which depend on this gadget.
>>
>> The use of -EFAULT for this case is similar to other systems, such
>> as FreeBSD, OpenBSD and Solaris.  QNX uses -EINVAL for this case.
>>
>> 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
>>
>> Changes from v1:
>> - Rework commit message significantly.
>> - Make the argv[0] check explicit rather than hijacking the error-check
>>   for count().
>>
>> Signed-off-by: Ariadne Conill <ariadne@dereferenced.org>
>> ---
>>  fs/exec.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/fs/exec.c b/fs/exec.c
>> index 79f2c9483302..e52c41991aab 100644
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -1899,6 +1899,10 @@ static int do_execveat_common(int fd, struct filename *filename,
>>  	retval = count(argv, MAX_ARG_STRINGS);
>>  	if (retval < 0)
>>  		goto out_free;
>> +	if (retval == 0) {
>> +		retval = -EFAULT;
>> +		goto out_free;
>> +	}
>>  	bprm->argc = retval;
>>
>>  	retval = count(envp, MAX_ARG_STRINGS);
>> --
>> 2.34.1
>
> Okay, so, the dangerous condition is userspace iterating through envp
> when it thinks it's iterating argv.
>
> Assuming it is not okay to break valgrind's test suite:
> https://sources.debian.org/src/valgrind/1:3.18.1-1/none/tests/execve.c/?hl=22#L22
> we cannot reject a NULL argv (test will fail), and we cannot mutate
> argc=0 into argc=1 (test will enter infinite loop).
>
> Perhaps we need to reject argv=NULL when envp!=NULL, and add a
> pr_warn_once() about using a NULL argv?

Sure, I can rework the patch to do it for only the envp != NULL case.

I think we should combine it with the {NULL, NULL} padding patch in this 
case though, since it appears to work, that way the execve(..., NULL, 
NULL) case gets some protection.

> I note that glibc already warns about NULL argv:
> argc0.c:7:3: warning: null argument where non-null required (argument 2)
> [-Wnonnull]
>    7 |   execve(argv[0], NULL, envp);
>      |   ^~~~~~
>
> in the future we could expand this to only looking at argv=NULL?

I don't think musl's headers generate a diagnostic for this, but main(0, 
{NULL}) is not a supported use-case at least as far as Alpine is 
concerned.  I am sure it is the same with the other musl distributions.

Will send a v3 patch with this logic change and move to EINVAL shortly.

Ariadne