Re: netlink: GPF in netlink_unicast

On 2017-03-07 14:23, Paul Moore wrote: > On Tue, Mar 7, 2017 at 1:44 PM, Paul Moore <paul(a)paul-moore.com&gt; wrote: > > On Tue, Mar 7, 2017 at 10:55 AM, Richard Guy Briggs <rgb(a)redhat.com&gt; wrote: > >> 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. > > > > 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 ... At the moment, you are right, there is no reason to null audit_sock, and not like auditd will appear on a different sock yet.

The only excuse I can give is that this was anticipating audit daemons in more than one user namespace necessarily with their own network namespaces. The AUDIT_GET, AUDIT_LIST_RULES commands are treated properly since they use the per-netns audit_net struct and don't use the primary queue. The AUDIT_USER_* messages are converted from their originating namespaces ok, but will need to be tracked what network namespace they came from for multiple audit daemons in the future. > Man I hate this code. I *really* hate this code. > > paul moore - RGB -- Richard Guy Briggs <rgb(a)redhat.com&gt; Kernel Security Engineering, Base Operating Systems, Red Hat Remote, Ottawa, Canada Voice: +1.647.777.2635, Internal: (81) 32635