Re: netlink: GPF in netlink_unicast

On 2017-03-07 09:29, Paul Moore wrote: > On Mon, Mar 6, 2017 at 11:03 PM, Richard Guy Briggs <rgb(a)redhat.com&gt; wrote: > > On 2017-03-06 10:10, Cong Wang wrote: > >> On Mon, Mar 6, 2017 at 2:54 AM, Dmitry Vyukov <dvyukov(a)google.com&gt; wrote: > >> > Hello, > >> > > >> > I've got the following crash while running syzkaller fuzzer on > >> > net-next/8d70eeb84ab277377c017af6a21d0a337025dede: > >> > > >> > kasan: GPF could be caused by NULL-ptr deref or user memory access > >> > general protection fault: 0000 [#1] SMP KASAN > >> > Dumping ftrace buffer: > >> > (ftrace buffer empty) > >> > Modules linked in: > >> > CPU: 0 PID: 883 Comm: kauditd Not tainted 4.10.0+ #6 > >> > Hardware name: Google Google Compute Engine/Google Compute Engine, > >> > BIOS Google 01/01/2011 > >> > task: ffff8801d79f0240 task.stack: ffff8801d7a20000 > >> > RIP: 0010:sock_sndtimeo include/net/sock.h:2162 [inline] > >> > RIP: 0010:netlink_unicast+0xdd/0x730 net/netlink/af_netlink.c:1249 > >> > RSP: 0018:ffff8801d7a27c38 EFLAGS: 00010206 > >> > RAX: 0000000000000056 RBX: ffff8801d7a27cd0 RCX: 0000000000000000 > >> > RDX: 0000000000000000 RSI: 0000000000000000 RDI: 00000000000002b0 > >> > RBP: ffff8801d7a27cf8 R08: ffffed00385cf286 R09: ffffed00385cf286 > >> > R10: 0000000000000006 R11: ffffed00385cf285 R12: 0000000000000000 > >> > R13: dffffc0000000000 R14: ffff8801c2fc3c80 R15: 00000000014000c0 > >> > FS: 0000000000000000(0000) GS:ffff8801dbe00000(0000) knlGS:0000000000000000 > >> > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > >> > CR2: 0000000020cfd000 CR3: 00000001c758f000 CR4: 00000000001406f0 > >> > Call Trace: > >> > kauditd_send_unicast_skb+0x3c/0x70 kernel/audit.c:482 > >> > kauditd_thread+0x174/0xb00 kernel/audit.c:599 > >> > kthread+0x326/0x3f0 kernel/kthread.c:229 > >> > ret_from_fork+0x31/0x40 arch/x86/entry/entry_64.S:430 > >> > Code: 44 89 fe e8 56 15 ff ff 8b 8d 70 ff ff ff 49 89 c6 31 c0 85 c9 > >> > 75 27 e8 b2 b2 f4 fd 49 8d bc 24 b0 02 00 00 48 89 f8 48 c1 e8 03 <42> > >> > 80 3c 28 00 0f 85 37 06 00 00 49 8b 84 24 b0 02 00 00 4c 8d > >> > RIP: sock_sndtimeo include/net/sock.h:2162 [inline] RSP: ffff8801d7a27c38 > >> > RIP: netlink_unicast+0xdd/0x730 net/netlink/af_netlink.c:1249 RSP: > >> > ffff8801d7a27c38 > >> > ---[ end trace ad1bba9d457430b6 ]--- > >> > Kernel panic - not syncing: Fatal exception > >> > > >> > > >> > This is not reproducible and seems to be caused by an elusive race. > >> > However, looking at the code I don't see any proper protection of > >> > audit_sock (other than the if (!audit_pid) which is obviously not > >> > enough to protect against races). > >> > >> audit_cmd_mutex is supposed to protect it, I think. > >> But kauditd_send_unicast_skb() seems not holding this mutex. > > > > Hmmmm, I wonder if it makes sense to wrap most of the contents of the > > outer while loop in kauditd_thread in the audit_cmd_mutex, or around the > > first two innter while loops and the "if (auditd)" condition after the > > "quick_loop:" label. The condition on auditd is supposed to catch that > > case. We don't want it locked while playing with the scheduler at the > > bottom of that function. > > Let me look into this and play around with a few things. I suspected > there might be a problem here, so I've got thoughts on how we might > resolve it; I just need to see code them up and see what option sucks > the least. > > FWIW Richard, yes wrapping most of kauditd_thread *should* resolve > this but it's pretty heavy handed and not my first choice. That's why the inner loops made a bit more sense since it wasn't really necessary and ran afoul of the scheduler anyways.