linux-kernel - Re: [PATCH] x86: panic when a kernel stack overflow is detected

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Date:   Sun, 28 Jul 2019 20:53:58 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Daniel Axtens <dja@...ens.net>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     kasan-dev <kasan-dev@...glegroups.com>, X86 ML <x86@...nel.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrew Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Marco Elver <elver@...gle.com>
Subject: Re: [PATCH] x86: panic when a kernel stack overflow is detected

On Sun, Jul 28, 2019 at 6:59 PM Daniel Axtens <dja@...ens.net> wrote:
>
> Currently, when a kernel stack overflow is detected via VMAP_STACK,
> the task is killed with die().
>
> This isn't safe, because we don't know how that process has affected
> kernel state. In particular, we don't know what locks have been taken.
> For example, we can hit a case with lkdtm where a thread takes a
> stack overflow in printk() after taking the logbuf_lock. In that case,
> we deadlock when the kernel next does a printk.
>
> Do not attempt to kill the process when a kernel stack overflow is
> detected. The system state is unknown, the only safe thing to do is
> panic(). (panic() also prints without taking locks so a useful debug
> splat is printed even when logbuf_lock is held.)

The thing I don't like about this is that it reduces the chance that
we successfully log anything to disk.

PeterZ, do you have any useful input here?  I wonder if we could do
something like printk_oh_crap() that is just printk() except that it
panics if it fails to return after a few seconds.

--Andy