Re: [syzbot] [wireguard?] KASAN: slab-use-after-free Write in enqueue_timer

[Eric Dumazet]

On Tue, May 23, 2023 at 7:01 PM Jason A. Donenfeld <Jason@xxxxxxxxx> 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 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?


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.