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

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Date:   Thu, 10 Feb 2022 09:37:36 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Jann Horn <jannh@...gle.com>
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
Subject: Re: [PATCH 1/3] signal: HANDLER_EXIT should clear SIGNAL_UNKILLABLE

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.

> 
> >         ret = send_signal(sig, info, t, PIDTYPE_PID);
> >         spin_unlock_irqrestore(&t->sighand->siglock, flags);
> > --
> > 2.30.2
> >

-- 
Kees Cook