linux-hardening - Re: [PATCH 1/3] signal: HANDLER_EXIT should clear SIGNAL

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Date:   Fri, 11 Feb 2022 21:15:07 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Robert Święcki <robert@...ecki.net>,
        stable@...r.kernel.org, Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH 1/3] signal: HANDLER_EXIT should clear SIGNAL_UNKILLABLE

On Thu, Feb 10, 2022 at 10:09 PM Kees Cook <keescook@...omium.org> wrote:
> On Thu, Feb 10, 2022 at 07:01:39PM +0100, Jann Horn wrote:
> > On Thu, Feb 10, 2022 at 6:37 PM Kees Cook <keescook@...omium.org> 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@...omium.org> 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@...ecki.net>
> > > > > Suggested-by: "Eric W. Biederman" <ebiederm@...ssion.com>
> > > > > Fixes: 00b06da29cf9 ("signal: Add SA_IMMUTABLE to ensure forced siganls do not get changed")
> > > > > Cc: stable@...r.kernel.org
> > > > > Signed-off-by: Kees Cook <keescook@...omium.org>
> > > > > ---
> > > > >  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?
>
> Do you mean after force_sig_info_to_task() has finished but before
> get_signal()? SA_IMMUTABLE will block changes to the action.

Yeah, that's what I meant.
Thanks, I missed SA_IMMUTABLE. Ugh, this is not pretty code...