Subject: Re: [PATCH bpf v6 03/12] bpf: sockmap, improved check for empty queue

From: John Fastabend <[email-masked]>
Date: Thu, 13 Apr 2023 13:06:45 -0700

Alexei Starovoitov wrote:
> On Fri, Apr 7, 2023 at 10:17 AM John Fastabend <[email-masked]> wrote:
> >
> > We noticed some rare sk_buffs were stepping past the queue when system was
> > under memory pressure. The general theory is to skip enqueueing
> > sk_buffs when its not necessary which is the normal case with a system
> > that is properly provisioned for the task, no memory pressure and enough
> > cpu assigned.
> >
> > But, if we can't allocate memory due to an ENOMEM error when enqueueing
> > the sk_buff into the sockmap receive queue we push it onto a delayed
> > workqueue to retry later. When a new sk_buff is received we then check
> > if that queue is empty. However, there is a problem with simply checking
> > the queue length. When a sk_buff is being processed from the ingress queue
> > but not yet on the sockmap msg receive queue its possible to also recv
> > a sk_buff through normal path. It will check the ingress queue which is
> > zero and then skip ahead of the pkt being processed.
> >
> > Previously we used sock lock from both contexts which made the problem
> > harder to hit, but not impossible.
> >
> > To fix also check the 'state' variable where we would cache partially
> > processed sk_buff. This catches the majority of cases. But, we also
> > need to use the mutex lock around this check because we can't have both
> > codes running and check sensibly. We could perhaps do this with atomic
> > bit checks, but we are already here due to memory pressure so slowing
> > things down a bit seems OK and simpler to just grab a lock.
> >
> > To reproduce issue we run NGINX compliance test with sockmap running and
> > observe some flakes in our testing that we attributed to this issue.
> >
> > Fixes: 04919bed948dc ("tcp: Introduce tcp_read_skb()")
> > Tested-by: William Findlay <[email-masked]>
> > Signed-off-by: John Fastabend <[email-masked]>
> > ---
> >  net/core/skmsg.c | 20 +++++++++++++++++++-
> >  1 file changed, 19 insertions(+), 1 deletion(-)
> >
> > diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> > index 198bed303c51..f8731818b5c3 100644
> > --- a/net/core/skmsg.c
> > +++ b/net/core/skmsg.c
> > @@ -987,6 +987,7 @@ EXPORT_SYMBOL_GPL(sk_psock_tls_strp_read);
> >  static int sk_psock_verdict_apply(struct sk_psock *psock, struct sk_buff *skb,
> >                                   int verdict)
> >  {
> > +       struct sk_psock_work_state *state;
> >         struct sock *sk_other;
> >         int err = 0;
> >         u32 len, off;
> > @@ -1003,13 +1004,28 @@ static int sk_psock_verdict_apply(struct sk_psock *psock, struct sk_buff *skb,
> >
> >                 skb_bpf_set_ingress(skb);
> >
> > +               /* We need to grab mutex here because in-flight skb is in one of
> > +                * the following states: either on ingress_skb, in psock->state
> > +                * or being processed by backlog and neither in state->skb and
> > +                * ingress_skb may be also empty. The troublesome case is when
> > +                * the skb has been dequeued from ingress_skb list or taken from
> > +                * state->skb because we can not easily test this case. Maybe we
> > +                * could be clever with flags and resolve this but being clever
> > +                * got us here in the first place and we note this is done under
> > +                * sock lock and backlog conditions mean we are already running
> > +                * into ENOMEM or other performance hindering cases so lets do
> > +                * the obvious thing and grab the mutex.
> > +                */
> > +               mutex_lock(&psock->work_mutex);
> > +               state = &psock->work_state;
> 
> This splat says that above is wrong:
> 
> [   98.732763] BUG: sleeping function called from invalid context at
> kernel/locking/mutex.c:580
> [   98.733483] in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid:
> 370, name: test_progs
> [   98.734103] preempt_count: 102, expected: 0
> [   98.734416] RCU nest depth: 4, expected: 0
> [   98.734739] 6 locks held by test_progs/370:
> [   98.735046]  #0: ffff888106475530 (sk_lock-AF_INET){+.+.}-{0:0},
> at: inet_shutdown+0x43/0x150
> [   98.735695]  #1: ffffffff84250ba0 (rcu_read_lock){....}-{1:2}, at:
> __ip_queue_xmit+0x5/0xa00
> [   98.736325]  #2: ffffffff84250ba0 (rcu_read_lock){....}-{1:2}, at:
> process_backlog+0xc0/0x360
> [   98.736971]  #3: ffffffff84250ba0 (rcu_read_lock){....}-{1:2}, at:
> ip_local_deliver_finish+0xbb/0x220
> [   98.737668]  #4: ffff8881064748b0 (slock-AF_INET/1){+.-.}-{2:2},
> at: tcp_v4_rcv+0x1b72/0x1d80
> [   98.738297]  #5: ffffffff84250ba0 (rcu_read_lock){....}-{1:2}, at:
> sk_psock_verdict_recv+0x5/0x3a0
> [   98.738973] Preemption disabled at:
> [   98.738976] [<ffffffff8238bb41>] ip_finish_output2+0x171/0xfa0
> [   98.739687] CPU: 1 PID: 370 Comm: test_progs Tainted: G           O
>       6.3.0-rc5-00193-g9149a3b041d2 #942
> [   98.740379] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
> BIOS rel-1.12.0-59-gc9ba5276e321-prebuilt.qemu.org 04/01/2014
> [   98.741164] Call Trace:
> [   98.741338]  <IRQ>
> [   98.741483]  dump_stack_lvl+0x60/0x70
> [   98.741735]  __might_resched+0x21c/0x340
> [   98.742005]  __mutex_lock+0xb4/0x12a0
> [   98.743721]  ? mutex_lock_io_nested+0x1070/0x1070
> [   98.744053]  ? lock_is_held_type+0xda/0x130
> [   98.744337]  ? preempt_count_sub+0x14/0xc0
> [   98.744624]  ? sk_psock_verdict_apply+0x1a3/0x2f0
> [   98.744936]  sk_psock_verdict_apply+0x1a3/0x2f0
> [   98.745242]  ? preempt_count_sub+0x14/0xc0
> [   98.745530]  sk_psock_verdict_recv+0x1e7/0x3a0
> [   98.745858]  ? preempt_count_sub+0x14/0xc0
> [   98.746168]  tcp_read_skb+0x19c/0x2d0
> [   98.746447]  ? sk_psock_strp_read+0x390/0x390
> [   98.746774]  ? tcp_alloc_md5sig_pool+0x230/0x230
> [   98.747116]  ? rcu_read_lock_held+0x91/0xa0
> [   98.747427]  ? rcu_read_lock_sched_held+0xc0/0xc0
> [   98.747772]  ? __rcu_read_unlock+0x6b/0x2a0
> [   98.748087]  sk_psock_verdict_data_ready+0x99/0x2d0
> [   98.748446]  tcp_data_queue+0xd39/0x19b0
> [   98.748749]  ? tcp_send_rcvq+0x280/0x280
> [   98.749038]  ? tcp_urg+0x7f/0x4c0
> [   98.749298]  ? tcp_ack_update_rtt.isra.55+0x910/0x910
> [   98.749644]  ? lockdep_hardirqs_on+0x79/0x100
> [   98.749940]  ? ktime_get+0x112/0x120
> [   98.750225]  ? ktime_get+0x86/0x120
> [   98.750498]  tcp_rcv_established+0x3fb/0xcc0
> [   98.752087]  tcp_v4_do_rcv+0x34a/0x4c0
> [   98.752400]  tcp_v4_rcv+0x1c9a/0x1d80
> [   98.754137]  ip_protocol_deliver_rcu+0x4f/0x4d0
> [   98.754562]  ip_local_deliver_finish+0x146/0x220
> [   98.754928]  ip_local_deliver+0x100/0x2e0
> [   98.756055]  ip_rcv+0xb6/0x2b0
> [   98.757689]  __netif_receive_skb_one_core+0xd2/0x110
> [   98.759121]  process_backlog+0x160/0x360
> [   98.759446]  __napi_poll+0x57/0x300
> [   98.759725]  net_rx_action+0x555/0x600
> [   98.760029]  ? napi_threaded_poll+0x2b0/0x2b0
> [   98.760444]  __do_softirq+0xeb/0x4e7
> [   98.760728]  ? ip_finish_output2+0x391/0xfa0
> [   98.761059]  do_softirq+0xa1/0xd0
> 
> I'm afraid I have to revert this set from bpf tree.

OK thanks. We also got a syzkaller complaint so will fix these two things.