Messages in this thread | | | From | Paul Moore <> | Date | Tue, 7 Mar 2017 14:23:52 -0500 | Subject | Re: netlink: GPF in netlink_unicast |
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On Tue, Mar 7, 2017 at 1:44 PM, Paul Moore <paul@paul-moore.com> wrote: > On Tue, Mar 7, 2017 at 10:55 AM, Richard Guy Briggs <rgb@redhat.com> wrote: >> On 2017-03-07 09:29, Paul Moore wrote: >>> On Mon, Mar 6, 2017 at 11:03 PM, Richard Guy Briggs <rgb@redhat.com> wrote: >>> > On 2017-03-06 10:10, Cong Wang wrote: >>> >> On Mon, Mar 6, 2017 at 2:54 AM, Dmitry Vyukov <dvyukov@google.com> 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. > > One of my preferred options was to get us away from protecting > everything with the audit_cmd_mutex by creating a new locking approach > for the auditd connection state (using RCU/spinlocks since it rarely > changes in practice) and leaving the audit_cmd_mutex for it's > traditional role. This should minimize the performance impact of the > lock and clean things up a bit. I'm also moving all the auditd > connection state into a single struct (instead of several variables > associated only by convention) which moves us oh so slightly closer to > allowing multiple auditd connections (hey, it's something). > > It's taking a bit longer than expected as I'm dealing with a bit of a > head cold (or something) and my mind is far less than 100% at the > moment ...
Ooof. I just noticed something, and maybe this is the fever talking, but why do we ever NULL out audit_sock and why are we bothering with those holds/puts? We create the audit netlink socket in audit_net_init() and it should remain valid until we kill it in audit_next_exit(); we sorta cheat on this now because we track the socket both in the per-netns audit_net struct as well as audit_sock, but that doesn't make our audit_sock manipulations right ...
Man I hate this code. I *really* hate this code.
-- paul moore www.paul-moore.com
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