On Thu, Mar 14, 2024 at 10:07 PM Josef Bacik <josef@xxxxxxxxxxxxxx> wrote:
>
> Hello,
>
> We've been hitting the following panic in production, and I've root caused
> what's happening, but I'm at a loss on how to fix it.
>
> The panic we're seeing is this
>
> BUG: kernel NULL pointer dereference, address: 0000000000000000
> RIP: 0010:ip6_pol_route+0x59/0x7a0
> Call Trace:
> <IRQ>
> ? __die+0x78/0xc0
> ? page_fault_oops+0x286/0x380
> ? fib6_table_lookup+0x95/0xf40
> ? exc_page_fault+0x5d/0x110
> ? asm_exc_page_fault+0x22/0x30
> ? ip6_pol_route+0x59/0x7a0
> ? unlink_anon_vmas+0x370/0x370
> fib6_rule_lookup+0x56/0x1b0
> ? update_blocked_averages+0x2c6/0x6a0
> ip6_route_output_flags+0xd2/0x130
> ip6_dst_lookup_tail+0x3b/0x220
> ip6_dst_lookup_flow+0x2c/0x80
> inet6_sk_rebuild_header+0x14c/0x1e0
> ? tcp_release_cb+0x150/0x150
> __tcp_retransmit_skb+0x68/0x6b0
> ? tcp_current_mss+0xca/0x150
> ? tcp_release_cb+0x150/0x150
> tcp_send_loss_probe+0x8e/0x220
> tcp_write_timer+0xbe/0x2d0
> run_timer_softirq+0x272/0x840
> ? hrtimer_interrupt+0x2c9/0x5f0
> ? sched_clock_cpu+0xc/0x170
> irq_exit_rcu+0x171/0x330
> sysvec_apic_timer_interrupt+0x6d/0x80
> </IRQ>
> <TASK>
> asm_sysvec_apic_timer_interrupt+0x16/0x20
> RIP: 0010:cpuidle_enter_state+0xe7/0x243
>
> Inspecting the vmcore with drgn you can see why this is a NULL pointer deref
>
> >>> prog.crashed_thread().stack_trace()[0]
> #0 at 0xffffffff810bfa89 (ip6_pol_route+0x59/0x796) in ip6_pol_route at net/ipv6/route.c:2212:40
>
> 2212 if (net->ipv6.devconf_all->forwarding == 0)
> 2213 strict |= RT6_LOOKUP_F_REACHABLE;
>
> >>> prog.crashed_thread().stack_trace()[0]['net'].ipv6.devconf_all
> (struct ipv6_devconf *)0x0
>
> Looking at the socket you can see that it's been closed
>
> >>> decode_enum_type_flags(prog.crashed_thread().stack_trace()[11]['sk'].__sk_common.skc_flags, prog.type('enum sock_flags'))
> 'SOCK_DEAD|SOCK_KEEPOPEN|SOCK_ZAPPED|SOCK_USE_WRITE_QUEUE'
> >>> decode_enum_type_flags(1 << prog.crashed_thread().stack_trace()[11]['sk'].__sk_common.skc_state.value_(), prog["TCPF_CLOSE"].type_, bit_numbers=False)
> 'TCPF_FIN_WAIT1'
>
> The way this reproduces is with our NFS setup. We have an NFS mount inside of a
> container, which has it's own network namespace. We setup the mount inside of
> this network namespace.
>
> On container shutdown sometimes we trigger this panic, it's pretty reliably
> reproduced, with a stress tier of 200 machines I can usually trigger it on ~10
> machines by stopping the jobs.
>
> My initial thought was that NFS wasn't properly shutting down the sockets, but
> this doesn't appear to be the case. The sock is always marked with SOCK_DEAD.
> My second thought was that we had some pending timers when we call
> kernel_sock_shutdown(), so I added tcp_clear_xmit_timers(sk); to tcp_shutdown()
> to make sure the timers were cleared. This didn't fix the issue.
>
> I added some debugging to the socket and flagged the socket when NFS called
> kernel_sock_shutdown() and then had a WARN_ON(sock_flag(sk,
> JOSEFS_SPECIAL_FLAG)) where we arm the timer, and that trips constantly. So
> we're definitely arming the sock after NFS has shutdown the socket.
>
> This is where we leave my ability to figure out what's going on and how to fix
> it. What seems to be happening is this
>
> 1. NFS calls kernel_sock_shutdown() when we unmount.
> 2. We get an ACK on the socket and the timer gets armed.
> 3. We shutdown the container and tear down the network namespace.
> 4. The timer fires and we try to send the loss probe and we panic because the
> network namespace teardown removes the devconf as part of its teardown.
>
> It appears to me that sock's will just hang around forever past the end of an
> application being done with it, tho I'm not sure if I'm correct in this. If
> that's the case then I don't know the correct way to handle this, other than
> adding an extra case for the timer to simply not run when SOCK_DEAD is set. But
> this seems to be done on purpose, so seems like that's a bad fix.
>
> Let me know if you have debug patches or other information you'd like from a
> vmcore, I have plenty. Like I said I can reproduce reliably, it does take a few
> hours to deploy a test kernel, but I can have a turn around of about a day for
> debug patches. Thanks,
>
> Josef
If NFS is using kernel sockets, it is NFS responsibility to remove
all of them when the netns is destroyed.
Also look at recent relevant patches
2a750d6a5b365265dbda33330a6188547ddb5c24 rds: tcp: Fix use-after-free
of net in reqsk_timer_handler().
1c4e97dd2d3c9a3e84f7e26346aa39bc426d3249 tcp: Fix NEW_SYN_RECV
handling in inet_twsk_purge()
