linux-kernel - pids_free double free.

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Date:	Thu, 19 Nov 2015 23:18:36 -0500
From:	Dave Jones <davej@...emonkey.org.uk>
To:	Linux Kernel <linux-kernel@...r.kernel.org>
Cc:	Tejun Heo <tj@...nel.org>
Subject: pids_free double free.

One of my debian boxes got a systemd update. After rebooting,
I started seeing a use-after-free trace, followed by a lockup.

I have two slightly different traces from separate boots,
which may give some clue as to how it's getting free'd in two ways..

http://codemonkey.org.uk/junk/IMG_0474.jpg
http://codemonkey.org.uk/junk/IMG_0476.jpg

This isn't new, I booted back to 4.3, and hit slab debugging warnings
from the same code.  (At the least it was broken differently)

The WARN_ON referenced in the 2nd trace is this..

static void pids_cancel(struct pids_cgroup *pids, int num)
{
        /*  
         * A negative count (or overflow for that matter) is invalid,
         * and indicates a bug in the `pids` controller proper.
         */
        WARN_ON_ONCE(atomic64_add_negative(-num, &pids->counter));
}

	Dave
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