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

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Date:   Thu, 10 Feb 2022 12:16:37 -0600
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     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

Kees Cook <keescook@...omium.org> writes:

> Fatal SIGSYS signals were not being delivered to pid namespace init
> processes . Make sure the SIGNAL_UNKILLABLE doesn't get set for these
           ^ when ptraced.
> cases.



Reviewed-by: "Eric W. Biederman" <ebiederm@...ssion.com>

I have pointed out a few nits in the wording but otherwise this looks good.
>
> 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.
perhaps                                     ^ HANDLER_EXIT is always fatal.
>  	 */
> -	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;
>  	ret = send_signal(sig, info, t, PIDTYPE_PID);
>  	spin_unlock_irqrestore(&t->sighand->siglock, flags);

Eric