linux-kernel - Re: [GIT PULL] sigqueue cache fix

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Date:   Mon, 28 Jun 2021 10:06:39 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] sigqueue cache fix

On Sun, Jun 27, 2021 at 10:14 PM Ingo Molnar <mingo@...nel.org> wrote:
>
> The most fundamental race we can have is this:

No. It's this (all on the same CPU):

   sigqueue_cache_or_free():

       if (!READ_ONCE(current->sigqueue_cache))
                     <-- Interrupt happens here
               WRITE_ONCE(current->sigqueue_cache, q);

and then the interrupt sends a SIGCONT, which ends up flushing
previous process control signals, which ends up freeing them, which
ends up in sigqueue_cache_or_free() again, at which point you have

       if (!READ_ONCE(current->sigqueue_cache))
               WRITE_ONCE(current->sigqueue_cache, q);

again.

And both the original and the interrupting one sees a NULL
current->sigqueue_cache, so both of them will do that WRITE_ONCE(),
and when the interrupt returns, the original case will overwrite the
value that the interrupt free'd.

Boom - memory leak.

It does seem to be very small race window, and it's "only" a memory
leak, but it's a very simple example of how this cache was broken even
on UP.

              Linus