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.
