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Date:   Tue, 23 May 2023 19:01:31 +0200
From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
To:     Jakub Kicinski <kuba@kernel.org>
Cc:     Eric Dumazet <edumazet@google.com>,
        syzbot <syzbot+c2775460db0e1c70018e@syzkaller.appspotmail.com>,
        netdev@vger.kernel.org, syzkaller-bugs@googlegroups.com,
        davem@davemloft.net, linux-kernel@vger.kernel.org,
        pabeni@redhat.com, wireguard@lists.zx2c4.com, jann@thejh.net
Subject: Re: [syzbot] [wireguard?] KASAN: slab-use-after-free Write in
 enqueue_timer
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On Tue, May 23, 2023 at 09:47:36AM -0700, Jakub Kicinski wrote:
> On Tue, 23 May 2023 18:42:53 +0200 Jason A. Donenfeld wrote:
> > > It should, no idea why it isn't. Looking thru the code now I don't see
> > > any obvious gaps where timer object is on a list but not active :S
> > > There's no way to get a vmcore from syzbot, right? :)
> > >
> > > Also I thought the shutdown leads to a warning when someone tries to
> > > schedule the dead timer but in fact add_timer() just exits cleanly.
> > > So the shutdown won't help us find the culprit :(  
> > 
> > Worth noting that it could also be caused by adding to a dead timer
> > anywhere in priv_data of another netdev, not just the sole timer_list
> > in net_device.
> 
> Oh, I thought you zero'ed in on the watchdog based on offsets.
> Still, object debug should track all timers in the slab and complain
> on the free path.

No, I mentioned watchdog because it's the only timer_list in struct
net_device.

Offset analysis is an interesting idea though. Look at this:

> The buggy address belongs to the object at ffff88801ecc0000
>  which belongs to the cache kmalloc-cg-8k of size 8192
> The buggy address is located 5376 bytes inside of
>  freed 8192-byte region [ffff88801ecc0000, ffff88801ecc2000)

IDA says that for syzkaller's vmlinux, net_device has a size of 0xc80
and wg_device has a size of 0x880. 0xc80+0x880=5376. Coincidence that
the address offset is just after what wg uses?

Hm.

Jason