Re: [PATCH v3] fs/exec: require argv[0] presence in do_execveat_common()

["Eric W. Biederman" <ebiederm@xmission.com>]

Kees Cook <keescook@chromium.org> writes:

> On Thu, Jan 27, 2022 at 12:07:24AM +0000, Ariadne Conill wrote:
>> In several other operating systems, it is a hard requirement that the
>> second 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 tool 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 execve(2) working
>> with argc < 1.
>> 
>> We use -EINVAL for this case, mirroring recent changes to FreeBSD and
>> OpenBSD.  -EINVAL is also used by QNX for this, while Solaris uses
>> -EFAULT.
>> 
>> In earlier versions of the patch, it was proposed that we create a
>> fake argv for applications to use when argc < 1, but it was concluded
>> that it would be better to just fail the execve(2) in these cases, as
>> launching a process with an empty or NULL argv[0] was likely to just
>> cause more problems.
>
> Let's do it and see what breaks. :)
>
> I do see at least tools/testing/selftests/exec/recursion-depth.c will
> need a fix. And maybe testcases/kernel/syscalls/execveat/execveat.h
> in LTP.
>
> Acked-by: Kees Cook <keescook@chromium.org>

Yes since this only appears to be tests that will break.

Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>

Especially since you are signing up to help fix the tests.


>> 
>> 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[2]
>> of this bug in a shellcode, we can reconsider.
>> 
>> This issue is being tracked in the KSPP issue tracker[3].
>> 
>> There are a few[4][5] minor edge cases (primarily in test suites) that
>> are caught by this, but we plan to work with the projects to fix those
>> edge cases.
>> 
>> [0]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html
>> [1]: https://bugzilla.kernel.org/show_bug.cgi?id=8408
>> [2]: https://www.qualys.com/2022/01/25/cve-2021-4034/pwnkit.txt
>> [3]: https://github.com/KSPP/linux/issues/176
>> [4]: https://codesearch.debian.net/search?q=execve%5C+*%5C%28%5B%5E%2C%5D%2B%2C+*NULL&literal=0
>> [5]: https://codesearch.debian.net/search?q=execlp%3F%5Cs*%5C%28%5B%5E%2C%5D%2B%2C%5Cs*NULL&literal=0
>> 
>> Changes from v2:
>> - Switch to using -EINVAL as the error code for this.
>> - Use pr_warn_once() to warn when an execve(2) is rejected due to NULL
>>   argv.
>> 
>> Changes from v1:
>> - Rework commit message significantly.
>> - Make the argv[0] check explicit rather than hijacking the error-check
>>   for count().
>> 
>> Reported-by: Michael Kerrisk <mtk.manpages@gmail.com>
>> To: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Matthew Wilcox <willy@infradead.org>
>> Cc: Christian Brauner <brauner@kernel.org>
>> Cc: Rich Felker <dalias@libc.org>
>> Cc: Eric Biederman <ebiederm@xmission.com>
>> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
>> Cc: Kees Cook <keescook@chromium.org>
>> Cc: linux-fsdevel@vger.kernel.org
>> Cc: linux-mm@kvack.org
>> Cc: stable@vger.kernel.org
>> 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..982730cfe3b8 100644
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -1897,6 +1897,10 @@ static int do_execveat_common(int fd, struct filename *filename,
>>  	}
>>  
>>  	retval = count(argv, MAX_ARG_STRINGS);
>> +	if (retval == 0) {
>> +		pr_warn_once("Attempted to run process '%s' with NULL argv\n", bprm->filename);
>> +		retval = -EINVAL;
>> +	}
>>  	if (retval < 0)
>>  		goto out_free;
>>  	bprm->argc = retval;
>> -- 
>> 2.34.1
>>