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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Wed, 24 May 2023 10:24:31 +0200
Message-ID: <CACT4Y+YcNt8rvJRK+XhCZa1Ocw9epHg1oSGc28mntjY3HWZp1g@mail.gmail.com>
Subject: Re: [syzbot] [wireguard?] KASAN: slab-use-after-free Write in enqueue_timer
To:     Eric Dumazet <edumazet@google.com>
Cc:     "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Jakub Kicinski <kuba@kernel.org>,
        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
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On Tue, 23 May 2023 at 19:07, 'Eric Dumazet' via syzkaller-bugs
<syzkaller-bugs@googlegroups.com> wrote:
>
> On Tue, May 23, 2023 at 7:05=E2=80=AFPM Eric Dumazet <edumazet@google.com=
> wrote:
> >
> > On Tue, May 23, 2023 at 7:01=E2=80=AFPM Jason A. Donenfeld <Jason@zx2c4=
.com> wrote:
> > >
> > > 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 do=
n'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 tri=
es to
> > > > > > schedule the dead timer but in fact add_timer() just exits clea=
nly.
> > > > > > So the shutdown won't help us find the culprit :(
> > > > >
> > > > > Worth noting that it could also be caused by adding to a dead tim=
er
> > > > > 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 complai=
n
> > > > 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=3D5376. Coincidence th=
at
> > > the address offset is just after what wg uses?
> >
> >
> > Note that the syzkaller report mentioned:
> >
> > alloc_netdev_mqs+0x89/0xf30 net/core/dev.c:10626
> >  usbnet_probe+0x196/0x2770 drivers/net/usb/usbnet.c:1698
> >  usb_probe_interface+0x5c4/0xb00 drivers/usb/core/driver.c:396
> >  really_probe+0x294/0xc30 drivers/base/dd.c:658
> >  __driver_probe_device+0x1a2/0x3d0 drivers/base/dd.c:800
> >  driver_probe_device+0x50/0x420 drivers/base/dd.c:830
> >  __device_attach_driver+0x2d3/0x520 drivers/base/dd.c:958
> >
> > So maybe an usbnet driver has a timer_list in its priv_data.
>
> struct usbnet {
> ...
> struct timer_list   delay;

FWIW There are more report examples on the dashboard.
There are some that don't mention wireguard nor usbnet, e.g.:
https://syzkaller.appspot.com/text?tag=3DCrashReport&x=3D17dd2446280000
So that's probably red herring. But they all seem to mention alloc_netdev_m=
qs.
Let's do for now:
#syz set subsystems: net