linux-kernel - Re: [syzbot] [wireguard?] KASAN: slab-use-after-free Write in enqueue

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Date:   Tue, 23 May 2023 19:28:54 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     edumazet@...gle.com,
        syzbot <syzbot+c2775460db0e1c70018e@...kaller.appspotmail.com>,
        netdev@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        davem@...emloft.net, linux-kernel@...r.kernel.org,
        pabeni@...hat.com, wireguard@...ts.zx2c4.com, jann@...jh.net
Subject: Re: [syzbot] [wireguard?] KASAN: slab-use-after-free Write in
 enqueue_timer

On Tue, May 23, 2023 at 07:16:20PM +0200, Jason A. Donenfeld wrote:
> On Tue, May 23, 2023 at 06:47:41PM +0200, Jason A. Donenfeld wrote:
> > On Tue, May 23, 2023 at 6:46 PM Jakub Kicinski <kuba@...nel.org> wrote:
> > >
> > > On Tue, 23 May 2023 18:14:18 +0200 Jason A. Donenfeld wrote:
> > > > So, IOW, not a wireguard bug, right?
> > >
> > > What's slightly concerning is that there aren't any other timers
> > > leading to
> > >
> > >   KASAN: slab-use-after-free Write in enqueue_timer
> > >
> > > :( If WG was just an innocent bystander there should be, right?
> > 
> > Well, WG does mod this timer for every single packet in its RX path.
> > So that's bound to turn things up I suppose.
> 
> Here's one that is seemingly the same -- enqueuing a timer to a freed
> base -- with the allocation and free being the same netdev core
> function, but the UaF trigger for it is a JBD2 transaction thing:
> https://syzkaller.appspot.com/text?tag=CrashReport&x=17dd2446280000
> No WG at all in it, but there's still the mysterious 5376 value...

In this one, you see the free happens in some infiniband code.  Looking
at ipoib_dev_priv, and going to the member at net_device+ipoib_dev_priv,
we get this at 5320:

        struct delayed_work        neigh_reap_task;

5376-5320=56, which doesn't quite put us at the timer_list. Close but no
cigar?