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

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Date:   Thu, 10 Feb 2022 17:18:39 +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
Subject: Re: [PATCH 1/3] signal: HANDLER_EXIT should clear SIGNAL_UNKILLABLE

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...

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