Subject: Re: [BUG bpf-next] bpf/net: Hitting gpf when running selftests

From: John Fastabend <[email-masked]>
Date: Thu, 31 Aug 2023 12:19:10 -0700

Xu Kuohai wrote:
> On 8/31/2023 5:46 PM, Xu Kuohai wrote:
> > On 8/31/2023 5:07 AM, Jiri Olsa wrote:
> >> On Wed, Aug 30, 2023 at 08:58:11PM +0200, Jiri Olsa wrote:
> >>> hi,
> >>> I'm hitting crash below on bpf-next/master when running selftests,
> >>> full log and config attached
> >>
> >> it seems to be 'test_progs -t sockmap_listen' triggering that
> >>
> >> jirka
> >>
> >>>
> >>> jirka
> >>>
> >>>
> >>> ---
> >>> [ 1022.710250][ T2556] general protection fault, probably for non-canonical address 0x6b6b6b6b6b6b6b73: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC NOPTI^M
> >>> [ 1022.711206][ T2556] CPU: 2 PID: 2556 Comm: kworker/2:4 Tainted: G           OE      6.5.0+ #693 1723c8b9805ff5a1672ab7e6f25977078a7bcceb^M
> >>> [ 1022.712120][ T2556] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-1.fc38 04/01/2014^M
> >>> [ 1022.712830][ T2556] Workqueue: events sk_psock_backlog^M
> >>> [ 1022.713262][ T2556] RIP: 0010:skb_dequeue+0x4c/0x80^M
> >>> [ 1022.713653][ T2556] Code: 41 48 85 ed 74 3c 8b 43 10 4c 89 e7 83 e8 01 89 43 10 48 8b 45 08 48 8b 55 00 48 c7 45 08 00 00 00 00 48 c7 45 00 00 00 00 00 <48> 89 42 08 48 89 10 e8 e8 6a 41 00 48 89 e8 5b 5d 41 5c c3 cc cc^M
> >>> [ 1022.714963][ T2556] RSP: 0018:ffffc90003ca7dd0 EFLAGS: 00010046^M
> >>> [ 1022.715431][ T2556] RAX: 6b6b6b6b6b6b6b6b RBX: ffff88811de269d0 RCX: 0000000000000000^M
> >>> [ 1022.716068][ T2556] RDX: 6b6b6b6b6b6b6b6b RSI: 0000000000000282 RDI: ffff88811de269e8^M
> >>> [ 1022.716676][ T2556] RBP: ffff888141ae39c0 R08: 0000000000000001 R09: 0000000000000000^M
> >>> [ 1022.717283][ T2556] R10: 0000000000000001 R11: 0000000000000000 R12: ffff88811de269e8^M
> >>> [ 1022.717930][ T2556] R13: 0000000000000001 R14: ffff888141ae39c0 R15: ffff88810a20e640^M
> >>> [ 1022.718549][ T2556] FS:  0000000000000000(0000) GS:ffff88846d600000(0000) knlGS:0000000000000000^M
> >>> [ 1022.719241][ T2556] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033^M
> >>> [ 1022.719761][ T2556] CR2: 00007fb5c25ca000 CR3: 000000012b902004 CR4: 0000000000770ee0^M
> >>> [ 1022.720394][ T2556] PKRU: 55555554^M
> >>> [ 1022.720699][ T2556] Call Trace:^M
> >>> [ 1022.720984][ T2556]  <TASK>^M
> >>> [ 1022.721254][ T2556]  ? die_addr+0x32/0x80^M
> >>> [ 1022.721589][ T2556]  ? exc_general_protection+0x25a/0x4b0^M
> >>> [ 1022.722026][ T2556]  ? asm_exc_general_protection+0x22/0x30^M
> >>> [ 1022.722489][ T2556]  ? skb_dequeue+0x4c/0x80^M
> >>> [ 1022.722854][ T2556]  sk_psock_backlog+0x27a/0x300^M
> >>> [ 1022.723243][ T2556]  process_one_work+0x2a7/0x5b0^M
> >>> [ 1022.723633][ T2556]  worker_thread+0x4f/0x3a0^M
> >>> [ 1022.723998][ T2556]  ? __pfx_worker_thread+0x10/0x10^M
> >>> [ 1022.724386][ T2556]  kthread+0xfd/0x130^M
> >>> [ 1022.724709][ T2556]  ? __pfx_kthread+0x10/0x10^M
> >>> [ 1022.725066][ T2556]  ret_from_fork+0x2d/0x50^M
> >>> [ 1022.725409][ T2556]  ? __pfx_kthread+0x10/0x10^M
> >>> [ 1022.725799][ T2556]  ret_from_fork_asm+0x1b/0x30^M
> >>> [ 1022.726201][ T2556]  </TASK>^M
> >>
> >>
> >> .
> > 
> > My patch failed on the BPF CI, and the log shows the test also died in skb_dequeue:
> > 
> > https://github.com/kernel-patches/bpf/actions/runs/6031993528/job/16366782122
> > 
> > [...]
> > 
> >    [   74.396478]  ? __die_body+0x1f/0x70
> >    [   74.396700]  ? page_fault_oops+0x15b/0x450
> >    [   74.396957]  ? fixup_exception+0x26/0x330
> >    [   74.397211]  ? exc_page_fault+0x68/0x1a0
> >    [   74.397457]  ? asm_exc_page_fault+0x26/0x30
> >    [   74.397724]  ? skb_dequeue+0x52/0x90
> >    [   74.397954]  sk_psock_destroy+0x8c/0x2b0
> >    [   74.398204]  process_one_work+0x28a/0x550
> >    [   74.398458]  ? __pfx_worker_thread+0x10/0x10
> >    [   74.398730]  worker_thread+0x51/0x3c0
> >    [   74.398966]  ? __pfx_worker_thread+0x10/0x10
> >    [   74.399235]  kthread+0xf7/0x130
> >    [   74.399437]  ? __pfx_kthread+0x10/0x10
> >    [   74.399707]  ret_from_fork+0x34/0x50
> >    [   74.399967]  ? __pfx_kthread+0x10/0x10
> >    [   74.400234]  ret_from_fork_asm+0x1b/0x30
> > 
> > 
> > After a few tries, I found a way to reproduce the problem.
> > 
> > Here is the reproduce steps:
> > 
> > 1. create a kprobe to delay sk_psock_backlog:
> > 
> > static struct kprobe kp = {
> >          .symbol_name = "sk_psock_backlog",
> >          .offset = 0x00,
> > };
> > 
> > static int handler_pre(struct kprobe *p, struct pt_regs *regs)
> > {
> >          mdelay(1000);
> >          return 0;
> > }
> > 
> > static int __init kprobe_init(void)
> > {
> >          int ret;
> > 
> >          kp.pre_handler = handler_pre;
> > 
> >          ret = register_kprobe(&kp);
> >          if (ret < 0) {
> >                  return -1;
> >          }
> > 
> >          return 0;
> > }
> > 
> > 2. insert the kprobe and run the vsock sockmap test:
> > 
> > ./test_progs -t "sockmap_listen/sockmap VSOCK test_vsock_redir"
> > 
> > 
> > 
> > I guess the problem is in sk_psock_backlog, where skb is inserted to another
> > list before skb_dequeue is called.
> > 
> > So I tested it with the following changes, and found the problem did go away.
> > 
> > --- a/net/core/skmsg.c
> > +++ b/net/core/skmsg.c
> > @@ -648,7 +648,7 @@ static void sk_psock_backlog(struct work_struct *work)
> >                  off = state->off;
> >          }
> > 
> > -       while ((skb = skb_peek(&psock->ingress_skb))) {
> > +       while ((skb = skb_dequeue(&psock->ingress_skb))) {
> >                  len = skb->len;
> >                  off = 0;
> >                  if (skb_bpf_strparser(skb)) {
> > @@ -684,7 +684,6 @@ static void sk_psock_backlog(struct work_struct *work)
> >                          len -= ret;
> >                  } while (len);
> > 
> > -               skb = skb_dequeue(&psock->ingress_skb);
> >                  if (!ingress) {
> >                          kfree_skb(skb);
> >                  }
> > 
> > Not clear what exactly happened, needs more debugging.
> >

I can only reproduce this on bpf-next so specific to
the vsock use case?

> 
> Use the skb address obtained from skb_peek() in sk_psock_backlog() as the key,
> 4 stack traces are obtained.
> 
> 
> trace 0, the skb is queued to the target socket ingress queue:
> 
> [  120.042016] sk_psock_skb_ingress_enqueue+0xf5/0x160
> [  120.045052] sk_psock_backlog+0x206/0x400
> [  120.047366] process_one_work+0x292/0x560
> [  120.049657] worker_thread+0x53/0x3e0
> [  120.051698] kthread+0x102/0x130
> [  120.053497] ret_from_fork+0x34/0x50
> [  120.055528] ret_from_fork_asm+0x1b/0x30
> 
> 
> trace 1, the skb is consumed by the user:
> 
> [  120.061537] consume_skb+0x47/0x100
> [  120.063394] sk_msg_recvmsg+0x268/0x3e0
> [  120.065458] unix_bpf_recvmsg+0x16c/0x610
> [  120.067584] unix_stream_recvmsg+0x66/0xa0
> [  120.069946] sock_recvmsg+0xc4/0xd0
> [  120.072063] __sys_recvfrom+0xaf/0x120
> [  120.073933] __x64_sys_recvfrom+0x29/0x30
> [  120.076052] do_syscall_64+0x3f/0x90
> [  120.077986] entry_SYSCALL_64_after_hwframe+0x6e/0xd8
> 
> trace 2, the vsock socket is closed by the user, and a new skb with
> the same address is allocated in vsock_release:
> 
> [  120.084296] __alloc_skb+0xe3/0x180
> [  120.086335] virtio_transport_alloc_skb+0x3b/0x2c0
> [  120.089174] virtio_transport_send_pkt_info+0x118/0x230
> [  120.092191] virtio_transport_release+0x29d/0x400
> [  120.094845] __vsock_release+0x3c/0x1e0
> [  120.096905] vsock_release+0x18/0x30
> [  120.098899] __sock_release+0x3d/0xc0
> [  120.100885] sock_close+0x18/0x20
> [  120.102606] __fput+0x108/0x2b0
> [  120.104636] task_work_run+0x5d/0xa0
> [  120.106876] exit_to_user_mode_prepare+0x18c/0x190
> [  120.109619] syscall_exit_to_user_mode+0x1d/0x50
> [  120.112049] do_syscall_64+0x4c/0x90
> [  120.114115] entry_SYSCALL_64_after_hwframe+0x6e/0xd8
> 
> trace 3, sk_psock_backlog() calls skb_dequeue() to unlink the skb, since
> this skb is now actually a new skb allocated in vsock_release, its prev
> and next fields are both set to NULL, NULL deref occurs.
> 
> [  120.120619] skb_dequeue+0x92/0xa0
> [  120.122350] sk_psock_backlog+0x305/0x400
> [  120.124512] process_one_work+0x292/0x560
> [  120.126771] worker_thread+0x53/0x3e0
> [  120.128843] kthread+0x102/0x130
> [  120.130772] ret_from_fork+0x34/0x50
> 
> To fix it, it seems reasonable to replace skb_peek() with skb_dequeue()
> in sk_psock_backlog(), since we can't prevent the skb from being appended
> to an ingress queue and consumed by user, as shown in trace 1 and trace 2.

The trouble with skb_dequeue is it breaks other checks that check
the backlog queue length. It really is nice to have a single len
check that determines if backlog is necessary or not.

If we revert something we likely need to go back to holding the
sock lock in backlog to ensure a reader can't eat the skb while
We still have a reference to it. It wasn't an issue for us because
its the exception case.

Trying to come up with some nice fix now.