Re: KASAN: use-after-free Write in detach_if_pending

[Eric Dumazet]

On Sun, 2017-10-29 at 13:45 +0100, Thomas Gleixner wrote:
> On Fri, 27 Oct 2017, syzbot wrote:
> 
> Cc'ed network folks.
> 
> > syzkaller hit the following crash on e7989f973ae1b90ec7c0b671c81f7f553affccbe
> > git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/master
> > compiler: gcc (GCC) 7.1.1 20170620
> > .config is attached
> > Raw console output is attached.
> > C reproducer is attached
> > syzkaller reproducer is attached. See https://goo.gl/kgGztJ
> > for information about syzkaller reproducers
> > 
> > 
> > BUG: KASAN: use-after-free in __write_once_size include/linux/compiler.h:305
> > [inline]
> > BUG: KASAN: use-after-free in __hlist_del include/linux/list.h:648 [inline]
> > BUG: KASAN: use-after-free in detach_timer kernel/time/timer.c:791 [inline]
> > BUG: KASAN: use-after-free in detach_if_pending+0x557/0x610
> > kernel/time/timer.c:808
> > Write of size 8 at addr ffff8801d3bab780 by task syzkaller900516/2986
> 
> That's just the point where this gets detected.
> 
> > CPU: 1 PID: 2986 Comm: syzkaller900516 Not tainted 4.13.0+ #82
> 
> > __hlist_del include/linux/list.h:648 [inline]
> > detach_timer kernel/time/timer.c:791 [inline]
> > detach_if_pending+0x557/0x610 kernel/time/timer.c:808
> > try_to_del_timer_sync+0xa2/0x120 kernel/time/timer.c:1182
> > del_timer_sync+0x18a/0x240 kernel/time/timer.c:1247
> > tun_flow_uninit drivers/net/tun.c:1104 [inline]
> > tun_free_netdev+0x105/0x1b0 drivers/net/tun.c:1776
> 
>   ^^^^^^^^^^^^ This shouldn't be called I think
> 
> > netdev_run_todo+0x870/0xca0 net/core/dev.c:7864
> > rtnl_unlock+0xe/0x10 net/core/rtnetlink.c:106
> > tun_detach drivers/net/tun.c:588 [inline]
> > tun_chr_close+0x49/0x60 drivers/net/tun.c:2609
> > __fput+0x333/0x7f0 fs/file_table.c:210
> > ____fput+0x15/0x20 fs/file_table.c:246
> > task_work_run+0x199/0x270 kernel/task_work.c:112
> > exit_task_work include/linux/task_work.h:21 [inline]
> > do_exit+0xa52/0x1b40 kernel/exit.c:865
> 
> Here is the allocation path
> 
> > alloc_netdev_mqs+0x16e/0xed0 net/core/dev.c:8018
> > tun_set_iff drivers/net/tun.c:2022 [inline]
> > __tun_chr_ioctl+0x12be/0x3d20 drivers/net/tun.c:2276
> > tun_chr_ioctl+0x2a/0x40 drivers/net/tun.c:2521
> > vfs_ioctl fs/ioctl.c:45 [inline]
> > do_vfs_ioctl+0x1b1/0x1530 fs/ioctl.c:685
> > SYSC_ioctl fs/ioctl.c:700 [inline]
> > SyS_ioctl+0x8f/0xc0 fs/ioctl.c:691
> > entry_SYSCALL_64_fastpath+0x1f/0xbe
> 
> 
> And this is free.
> 
> > netdev_freemem net/core/dev.c:7970 [inline]
> > free_netdev+0x2cf/0x360 net/core/dev.c:8132
> > tun_set_iff drivers/net/tun.c:2105 [inline]
> 
> err_free_flow:
>         tun_flow_uninit(tun);	<--------
> 
> > __tun_chr_ioctl+0x2cf6/0x3d20 drivers/net/tun.c:2276
> > tun_chr_ioctl+0x2a/0x40 drivers/net/tun.c:2521
> > vfs_ioctl fs/ioctl.c:45 [inline]
> > do_vfs_ioctl+0x1b1/0x1530 fs/ioctl.c:685
> > SYSC_ioctl fs/ioctl.c:700 [inline]
> > SyS_ioctl+0x8f/0xc0 fs/ioctl.c:691
> > entry_SYSCALL_64_fastpath+0x1f/0xbe
> 
> So it's the TUNSETIFF ioctl which first allocates and then frees in the
> errorpath of tun_set_iff.
> 
> But for some reason this sticks and the exit of that task does it again,
> which triggers KASAN in the innocent timer code.

Pretty old story, already fixed in David Miller trees.

net-next tree :

$ git log --oneline e7989f973ae1b90ec7c0b671c81.. -- drivers/net/tun.c
f8ddadc4db6c7b7029b6d0e0d9af24f74ad27ca2 Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net
ee74d9967b829232723939cb7c9b100b29f6ec98 tun: do not arm flow_gc_timer in tun_flow_init()
81d98fa4df3d1683b3ef21e8a7a0ccac7874f0de tun: avoid extra timer schedule in tun_flow_cleanup()
7dbfb4ef77db5666f0f3a425e7db93ca30ff4285 tun: do not block BH again in tun_flow_cleanup()
aec72f3392b1d598a979e89c4fdb131965ae0ab3 net-tun: fix panics at dismantle time
010f245b9dd734adda6386c494a4ace953ea8dc4 tun: relax check on eth_get_headlen() return value
0ad646c81b2182f7fa67ec0c8c825e0ee165696d tun: call dev_get_valid_name() before register_netdevice()
53954cf8c5d205624167a2bfd117cc0c1a5f3c6d Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net
2580c4c17aee3ad58e9751012bad278dd074ccae tun: bail out from tun_get_user() if the skb is empty
de8f3a83b0a0fddb2cf56e7a718127e9619ea3da bpf: add meta pointer for direct access
9484dc74fcf0750cd6726c9aa27edf97223916a8 tun: delete original tun_get() and rename __tun_get() to tun_get()
90e33d45940793def6f773b2d528e9f3c84ffdc7 tun: enable napi_gro_frags() for TUN/TAP driver
943170998b200190f99d3fe7e771437e2c51f319 tun: enable NAPI for TUN/TAP driver

net tree :

$ git log --oneline e7989f973ae1b90ec7c0b671c81.. -- drivers/net/tun.c
63b9ab65bd76e5de6479bb14b4014b64aa1a317a tuntap: properly align skb->head before building skb
5c25f65fd1e42685f7ccd80e0621829c105785d9 tun: allow positive return values on dev_get_valid_name() call
0ad646c81b2182f7fa67ec0c8c825e0ee165696d tun: call dev_get_valid_name() before register_netdevice()
2580c4c17aee3ad58e9751012bad278dd074ccae tun: bail out from tun_get_user() if the skb is empty

Pick the fixes, they are at least 2 patches that addressed the issue.