Re: [PATCH 13/20] signal: Implement force_fatal_sig

Andy Lutomirski
2021-10-25T22:41:04Z

On 10/20/21 13:05, Linus Torvalds wrote:
> On Wed, Oct 20, 2021 at 7:45 AM Eric W. Biederman <ebiederm@xmission.com> wrote:
>>
>> Add a simple helper force_fatal_sig that causes a signal to be
>> delivered to a process as if the signal handler was set to SIG_DFL.
>>
>> Reimplement force_sigsegv based upon this new helper.
>
> Can you just make the old force_sigsegv() go away? The odd special
> casing of SIGSEGV was odd to begin with, I think everybody really just
> wanted this new "force_fatal_sig()" and allow any signal - not making
> SIGSEGV special.
>

I'm rather nervous about all this, and I'm also nervous about the
existing code. A quick skim is finding plenty of code paths that assume
force_sigsegv (or a do_exit that this series touches) are genuinely
unrecoverable. For example:

- rseq: the *kernel* will be fine if a signal is handled, but the
userspace process may be in a very strange state.

- bprm_execve: The comment says it best:

/*
* If past the point of no return ensure the code never
* returns to the userspace process. Use an existing fatal
* signal if present otherwise terminate the process with
* SIGSEGV.
*/
if (bprm->point_of_no_return && !fatal_signal_pending(current))
force_sigsegv(SIGSEGV);

- vm86: already discussed

Now force_sigsegv() at least tries to kill the task, but not very well.
With the whole series applied and force_sigsegv() gone, these errors
become handleable, and that needs real care.

(I don't think bprm_execve() is exploitable. It looks like it's
attackable in the window between setting point_of_no_return and
unshare_sighand(), but I'm not seeing any useful way to attack it unless
a core dump is already in progress or a *different* fatal signal is
already pending, and in either of those cases we're fine.)