linux-kernel - Re: [PATCH 2/2] signal: Better detection of synchronous signals

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Date:   Mon, 11 Feb 2019 16:18:40 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        jolsa@...hat.com, Namhyung Kim <namhyung@...nel.org>,
        luca abeni <luca.abeni@...tannapisa.it>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: [PATCH 2/2] signal: Better detection of synchronous signals

Eric, I'll try to finally read the whole thread later, probably I missed
something, but...

On 02/07, Eric W. Biederman wrote:
>
> Recently syzkaller was able to create unkillablle processes by
> creating a timer that is delivered as a thread local signal on SIGHUP,
> and receiving SIGHUP SA_NODEFERER.  Ultimately causing a loop failing
> to deliver SIGHUP but always trying.
>
> When the stack overflows delivery of SIGHUP fails and force_sigsegv is
> called.  Unfortunately because SIGSEGV is numerically higher than
> SIGHUP next_signal tries again to deliver a SIGHUP.

Confused... In this particular case next_signal() should return SIGSEGV
because it must be pending too and SYNCHRONOUS_MASK doesn't include SIGHUP.

Not that it really matters, the timer can deliver another SYNCHRONOUS_MASK
signal < SIGSEGV, just I am trying to understand what have I missed...

> +	/*
> +	 * Check if there is another siginfo for the same signal.
> +	 */
> +	list_for_each_entry_continue(q, &pending->list, list) {
> +		if (q->info.si_signo == sync->info.si_signo)
> +			goto still_pending;
> +	}

But this must not be possible? SYNCHRONOUS_MASK doesn't include real-time
signals, we can't have 2 siginfo's for the same signal < SIGRTMIN.

Oleg.