Re: [PATCH] signal: Add SA_IMMUTABLE to ensure forced siganls do not get changed

Eric W. Biederman
2021-11-01T22:28:19Z

Andrea Righi <andrea.righi@canonical.com> writes:

> On Fri, Oct 29, 2021 at 10:09:04AM -0500, Eric W. Biederman wrote:
>>
>> As Andy pointed out that there are races between
>> force_sig_info_to_task and sigaction[1] when force_sig_info_task. As
>> Kees discovered[2] ptrace is also able to change these signals.
>>
>> In the case of seeccomp killing a process with a signal it is a
>> security violation to allow the signal to be caught or manipulated.
>>
>> Solve this problem by introducing a new flag SA_IMMUTABLE that
>> prevents sigaction and ptrace from modifying these forced signals.
>> This flag is carefully made kernel internal so that no new ABI is
>> introduced.
>>
>> Longer term I think this can be solved by guaranteeing short circuit
>> delivery of signals in this case. Unfortunately reliable and
>> guaranteed short circuit delivery of these signals is still a ways off
>> from being implemented, tested, and merged. So I have implemented a much
>> simpler alternative for now.
>>
>> [1] https://lkml.kernel.org/r/b5d52d25-7bde-4030-a7b1-7c6f8ab90660@www.fastmail.com
>> [2] https://lkml.kernel.org/r/202110281136.5CE65399A7@keescook
>> Cc: stable@vger.kernel.org
>> Fixes: 307d522f5eb8 ("signal/seccomp: Refactor seccomp signal and coredump generation")
>> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
>> ---
>
> FWIW I've tested this patch and I confirm that it fixes the failure that
> I reported with the seccomp_bpf selftest.
>
> Tested-by: Andrea Righi <andrea.righi@canonical.com>

Sigh. Except for the extra 0 in the definition of SA_IMMUTABLE
that caused it to conflict with the x86 specific signal numbers.

Eric