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Date:   Wed, 26 Jan 2022 11:41:05 -0600 (CST)
From:   Ariadne Conill <ariadne@dereferenced.org>
To:     Matthew Wilcox <willy@infradead.org>
cc:     Ariadne Conill <ariadne@dereferenced.org>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        Eric Biederman <ebiederm@xmission.com>,
        Kees Cook <keescook@chromium.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH v2] fs/exec: require argv[0] presence in
 do_execveat_common()
In-Reply-To: <YfFddcQyHHofTwgg@casper.infradead.org>
Message-ID: <15915be5-49c-77ba-d9f9-cf612f8211cb@dereferenced.org>
References: <20220126114447.25776-1-ariadne@dereferenced.org> <YfFddcQyHHofTwgg@casper.infradead.org>
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Hi,

On Wed, 26 Jan 2022, Matthew Wilcox 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;
>> +	}
>
> I don't object to the concept, but it's a more common pattern in Linux
> to do this:
>
> 	retval = count(argv, MAX_ARG_STRINGS);
> +	if (retval == 0)
> +		retval = -EFAULT;
> 	if (retval < 0)
> 		goto out_free;

Yeah, that seems fine.  We can of course do it that way, which I will 
revise the patch to do if we decide to stick with denial over making a 
"safe" argv instead.

> (aka I like my bikesheds painted in Toasty Eggshell)

Toasty Eggshell is a nice color for a bikeshed :)

Ariadne