Re: [PATCH] exec: Force single empty string when argv is empty

[Kees Cook <keescook@chromium.org>]

On February 1, 2022 6:53:25 AM PST, Rich Felker <dalias@libc.org> wrote:
>On Mon, Jan 31, 2022 at 04:09:47PM -0800, Kees Cook wrote:
>> Quoting[1] Ariadne Conill:
>> 
>> "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[2]:
>> 
>>     The argument arg0 should point to a filename string that is
>>     associated with the process being started by one of the exec
>>     functions.
>> ....
>> Interestingly, Michael Kerrisk opened an issue about this in 2008[3],
>> but there was no consensus to support fixing this issue then.
>> Hopefully now that CVE-2021-4034 shows practical exploitative use[4]
>> of this bug in a shellcode, we can reconsider.
>> 
>> This issue is being tracked in the KSPP issue tracker[5]."
>> 
>> While the initial code searches[6][7] turned up what appeared to be
>> mostly corner case tests, trying to that just reject argv == NULL
>> (or an immediately terminated pointer list) quickly started tripping[8]
>> existing userspace programs.
>> 
>> The next best approach is forcing a single empty string into argv and
>> adjusting argc to match. The number of programs depending on argc == 0
>> seems a smaller set than those calling execve with a NULL argv.
>> 
>> Account for the additional stack space in bprm_stack_limits(). Inject an
>> empty string when argc == 0 (and set argc = 1). Warn about the case so
>> userspace has some notice about the change:
>> 
>>     process './argc0' launched './argc0' with NULL argv: empty string added
>> 
>> Additionally WARN() and reject NULL argv usage for kernel threads.
>> 
>> [1] https://lore.kernel.org/lkml/20220127000724.15106-1-ariadne@dereferenced.org/
>> [2] https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html
>> [3] https://bugzilla.kernel.org/show_bug.cgi?id=8408
>> [4] https://www.qualys.com/2022/01/25/cve-2021-4034/pwnkit.txt
>> [5] https://github.com/KSPP/linux/issues/176
>> [6] https://codesearch.debian.net/search?q=execve%5C+*%5C%28%5B%5E%2C%5D%2B%2C+*NULL&literal=0
>> [7] https://codesearch.debian.net/search?q=execlp%3F%5Cs*%5C%28%5B%5E%2C%5D%2B%2C%5Cs*NULL&literal=0
>> [8] https://lore.kernel.org/lkml/20220131144352.GE16385@xsang-OptiPlex-9020/
>> 
>> Reported-by: Ariadne Conill <ariadne@dereferenced.org>
>> Reported-by: Michael Kerrisk <mtk.manpages@gmail.com>
>> 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: linux-fsdevel@vger.kernel.org
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> ---
>>  fs/exec.c | 26 +++++++++++++++++++++++++-
>>  1 file changed, 25 insertions(+), 1 deletion(-)
>> 
>> diff --git a/fs/exec.c b/fs/exec.c
>> index 79f2c9483302..bbf3aadf7ce1 100644
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -495,8 +495,14 @@ static int bprm_stack_limits(struct linux_binprm *bprm)
>>  	 * the stack. They aren't stored until much later when we can't
>>  	 * signal to the parent that the child has run out of stack space.
>>  	 * Instead, calculate it here so it's possible to fail gracefully.
>> +	 *
>> +	 * In the case of argc = 0, make sure there is space for adding a
>> +	 * empty string (which will bump argc to 1), to ensure confused
>> +	 * userspace programs don't start processing from argv[1], thinking
>> +	 * argc can never be 0, to keep them from walking envp by accident.
>> +	 * See do_execveat_common().
>>  	 */
>> -	ptr_size = (bprm->argc + bprm->envc) * sizeof(void *);
>> +	ptr_size = (min(bprm->argc, 1) + bprm->envc) * sizeof(void *);
>
>From #musl:
>
><mixi> kees: shouldn't the min(bprm->argc, 1) be max(...) in your patch?

Fix has already been sent, yup.

>I'm pretty sure without fixing that, you're introducing a giant vuln
>here.

I wouldn't say "giant", but yes, it weakened a defense in depth for avoiding high stack utilization.

> I believe this is the second time a patch attempting to fix this
>non-vuln has proposed adding a new vuln...

Mistakes happen, and that's why there is review and testing. Thank you for being part of the review process! :)

-Kees

-- 
Kees Cook