Re: [PATCH 1/3] signal: HANDLER_EXIT should clear SIGNAL_UNKILLABLE

[Jann Horn]

On Thu, Feb 10, 2022 at 6:37 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Thu, Feb 10, 2022 at 05:18:39PM +0100, Jann Horn wrote:
> > On Thu, Feb 10, 2022 at 3:53 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> > > Fatal SIGSYS signals were not being delivered to pid namespace init
> > > processes. Make sure the SIGNAL_UNKILLABLE doesn't get set for these
> > > cases.
> > >
> > > Reported-by: Robert Święcki <robert@xxxxxxxxxxx>
> > > Suggested-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> > > Fixes: 00b06da29cf9 ("signal: Add SA_IMMUTABLE to ensure forced siganls do not get changed")
> > > Cc: stable@xxxxxxxxxxxxxxx
> > > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > > ---
> > >  kernel/signal.c | 5 +++--
> > >  1 file changed, 3 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/kernel/signal.c b/kernel/signal.c
> > > index 38602738866e..33e3ee4f3383 100644
> > > --- a/kernel/signal.c
> > > +++ b/kernel/signal.c
> > > @@ -1342,9 +1342,10 @@ force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t,
> > >         }
> > >         /*
> > >          * Don't clear SIGNAL_UNKILLABLE for traced tasks, users won't expect
> > > -        * debugging to leave init killable.
> > > +        * debugging to leave init killable, unless it is intended to exit.
> > >          */
> > > -       if (action->sa.sa_handler == SIG_DFL && !t->ptrace)
> > > +       if (action->sa.sa_handler == SIG_DFL &&
> > > +           (!t->ptrace || (handler == HANDLER_EXIT)))
> > >                 t->signal->flags &= ~SIGNAL_UNKILLABLE;
> >
> > You're changing the subclause:
> >
> > !t->ptrace
> >
> > to:
> >
> > (!t->ptrace || (handler == HANDLER_EXIT))
> >
> > which means that the change only affects cases where the process has a
> > ptracer, right? That's not the scenario the commit message is talking
> > about...
>
> Sorry, yes, I was not as accurate as I should have been in the commit
> log. I have changed it to:
>
> Fatal SIGSYS signals (i.e. seccomp RET_KILL_* syscall filter actions)
> were not being delivered to ptraced pid namespace init processes. Make
> sure the SIGNAL_UNKILLABLE doesn't get set for these cases.

So basically force_sig_info() is trying to figure out whether
get_signal() will later on check for SIGNAL_UNKILLABLE (the SIG_DFL
case), and if so, it clears the flag from the target's signal_struct
that marks the process as unkillable?

This used to be:

if (action->sa.sa_handler == SIG_DFL)
    t->signal->flags &= ~SIGNAL_UNKILLABLE;

Then someone noticed that in the ptrace case, the signal might not
actually end up being consumed by the target process, and added the
"&& !t->ptrace" clause in commit
eb61b5911bdc923875cde99eb25203a0e2b06d43.

And now Robert Swiecki noticed that that still didn't accurately model
what'll happen in get_signal().


This seems hacky to me, and also racy: What if, while you're going
through a SECCOMP_RET_KILL_PROCESS in an unkillable process, some
other thread e.g. concurrently changes the disposition of SIGSYS from
a custom handler to SIG_DFL?

Instead of trying to figure out whether the signal would have been
fatal without SIGNAL_UNKILLABLE, I think it would be better to find a
way to tell the signal-handling code that SIGNAL_UNKILLABLE should be
bypassed for this specific signal, or something along those lines...
but of course that's also kind of messy because the signal-sending
code might fall back to just using the pending signal mask on
allocation failure IIRC?