[syzbot] [virt?] KCSAN: data-race in virtqueue_disable_cb / vring_interrupt (4)

[syzbot] [virt?] KCSAN: data-race in virtqueue_disable_cb / vring_interrupt (4)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    7c6a3a65ace7 minmax: reduce min/max macro expansion in ato..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1608e49f980000
kernel config:  https://syzkaller.appspot.com/x/.config?x=1e7d02549be622b2
dashboard link: https://syzkaller.appspot.com/bug?extid=8a02104389c2e0ef5049
compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/a1f7496fa21f/disk-7c6a3a65.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/f423739e51a9/vmlinux-7c6a3a65.xz
kernel image: https://storage.googleapis.com/syzbot-assets/b65a0f38cbd7/bzImage-7c6a3a65.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+8a02104389c2e0ef5049@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in virtqueue_disable_cb / vring_interrupt

write to 0xffff88810285ef52 of 1 bytes by interrupt on cpu 0:
 vring_interrupt+0x12b/0x180 drivers/virtio/virtio_ring.c:2591
 __handle_irq_event_percpu+0x95/0x490 kernel/irq/handle.c:158
 handle_irq_event_percpu kernel/irq/handle.c:193 [inline]
 handle_irq_event+0x64/0xf0 kernel/irq/handle.c:210
 handle_edge_irq+0x16d/0x5b0 kernel/irq/chip.c:831
 generic_handle_irq_desc include/linux/irqdesc.h:173 [inline]
 handle_irq arch/x86/kernel/irq.c:247 [inline]
 call_irq_handler arch/x86/kernel/irq.c:259 [inline]
 __common_interrupt+0x58/0xe0 arch/x86/kernel/irq.c:285
 common_interrupt+0x7c/0x90 arch/x86/kernel/irq.c:278
 asm_common_interrupt+0x26/0x40 arch/x86/include/asm/idtentry.h:693
 __preempt_count_dec_and_test arch/x86/include/asm/preempt.h:94 [inline]
 __raw_spin_unlock_irq include/linux/spinlock_api_smp.h:160 [inline]
 _raw_spin_unlock_irq+0x2f/0x50 kernel/locking/spinlock.c:202
 process_one_work kernel/workqueue.c:3200 [inline]
 process_scheduled_works+0x43b/0x9a0 kernel/workqueue.c:3312
 worker_thread+0x526/0x6e0 kernel/workqueue.c:3389
 kthread+0x1d1/0x210 kernel/kthread.c:389
 ret_from_fork+0x4b/0x60 arch/x86/kernel/process.c:147
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244

read to 0xffff88810285ef52 of 1 bytes by interrupt on cpu 1:
 virtqueue_disable_cb_split drivers/virtio/virtio_ring.c:892 [inline]
 virtqueue_disable_cb+0x92/0x180 drivers/virtio/virtio_ring.c:2446
 start_xmit+0x14b/0x1260 drivers/net/virtio_net.c:3034
 __netdev_start_xmit include/linux/netdevice.h:4913 [inline]
 netdev_start_xmit include/linux/netdevice.h:4922 [inline]
 xmit_one net/core/dev.c:3580 [inline]
 dev_hard_start_xmit+0x119/0x3f0 net/core/dev.c:3596
 sch_direct_xmit+0x1b0/0x580 net/sched/sch_generic.c:343
 __dev_xmit_skb net/core/dev.c:3809 [inline]
 __dev_queue_xmit+0xf3d/0x1fe0 net/core/dev.c:4389
 dev_queue_xmit include/linux/netdevice.h:3105 [inline]
 neigh_hh_output include/net/neighbour.h:526 [inline]
 neigh_output include/net/neighbour.h:540 [inline]
 ip_finish_output2+0x73d/0x8b0 net/ipv4/ip_output.c:235
 ip_finish_output+0x11a/0x2a0 net/ipv4/ip_output.c:323
 NF_HOOK_COND include/linux/netfilter.h:303 [inline]
 ip_output+0xab/0x170 net/ipv4/ip_output.c:433
 dst_output include/net/dst.h:450 [inline]
 ip_local_out net/ipv4/ip_output.c:129 [inline]
 __ip_queue_xmit+0xbcc/0xbf0 net/ipv4/ip_output.c:535
 ip_queue_xmit+0x38/0x50 net/ipv4/ip_output.c:549
 __tcp_transmit_skb+0x15b0/0x1970 net/ipv4/tcp_output.c:1466
 tcp_transmit_skb net/ipv4/tcp_output.c:1484 [inline]
 tcp_write_xmit+0x1273/0x31c0 net/ipv4/tcp_output.c:2829
 __tcp_push_pending_frames+0x6a/0x1a0 net/ipv4/tcp_output.c:3014
 tcp_push_pending_frames include/net/tcp.h:2106 [inline]
 tcp_data_snd_check net/ipv4/tcp_input.c:5707 [inline]
 tcp_rcv_established+0xa12/0xef0 net/ipv4/tcp_input.c:6141
 tcp_v4_do_rcv+0x662/0x740 net/ipv4/tcp_ipv4.c:1911
 tcp_v4_rcv+0x1b25/0x1ee0 net/ipv4/tcp_ipv4.c:2346
 ip_protocol_deliver_rcu+0x370/0x720 net/ipv4/ip_input.c:205
 ip_local_deliver_finish+0x17d/0x210 net/ipv4/ip_input.c:233
 NF_HOOK include/linux/netfilter.h:314 [inline]
 ip_local_deliver+0xec/0x1d0 net/ipv4/ip_input.c:254
 dst_input include/net/dst.h:460 [inline]
 ip_sublist_rcv_finish net/ipv4/ip_input.c:580 [inline]
 ip_list_rcv_finish net/ipv4/ip_input.c:631 [inline]
 ip_sublist_rcv+0x55a/0x640 net/ipv4/ip_input.c:639
 ip_list_rcv+0x25e/0x290 net/ipv4/ip_input.c:674
 __netif_receive_skb_list_ptype net/core/dev.c:5704 [inline]
 __netif_receive_skb_list_core+0x4f0/0x520 net/core/dev.c:5752
 __netif_receive_skb_list net/core/dev.c:5804 [inline]
 netif_receive_skb_list_internal+0x4e4/0x660 net/core/dev.c:5896
 gro_normal_list include/net/gro.h:515 [inline]
 napi_complete_done+0x1cb/0x450 net/core/dev.c:6247
 virtqueue_napi_complete drivers/net/virtio_net.c:694 [inline]
 virtnet_poll+0x1b73/0x1fd0 drivers/net/virtio_net.c:2826
 __napi_poll+0x63/0x3c0 net/core/dev.c:6772
 napi_poll net/core/dev.c:6841 [inline]
 net_rx_action+0x3a1/0x7f0 net/core/dev.c:6963
 handle_softirqs+0xc3/0x280 kernel/softirq.c:554
 __do_softirq kernel/softirq.c:588 [inline]
 invoke_softirq kernel/softirq.c:428 [inline]
 __irq_exit_rcu kernel/softirq.c:637 [inline]
 irq_exit_rcu+0x3e/0x90 kernel/softirq.c:649
 instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1043 [inline]
 sysvec_apic_timer_interrupt+0x37/0x80 arch/x86/kernel/apic/apic.c:1043
 asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:702

value changed: 0x00 -> 0x01

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 31238 Comm: syz.2.3839 Not tainted 6.11.0-rc7-syzkaller-00021-g7c6a3a65ace7 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/06/2024
==================================================================


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup

Re: [syzbot] [virt?] KCSAN: data-race in virtqueue_disable_cb / vring_interrupt (4)

[Michael S. Tsirkin]

On Thu, Sep 12, 2024 at 01:11:21AM -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    7c6a3a65ace7 minmax: reduce min/max macro expansion in ato..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1608e49f980000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=1e7d02549be622b2
> dashboard link: https://syzkaller.appspot.com/bug?extid=8a02104389c2e0ef5049
> compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/a1f7496fa21f/disk-7c6a3a65.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/f423739e51a9/vmlinux-7c6a3a65.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/b65a0f38cbd7/bzImage-7c6a3a65.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+8a02104389c2e0ef5049@syzkaller.appspotmail.com
> 
> ==================================================================
> BUG: KCSAN: data-race in virtqueue_disable_cb / vring_interrupt
> 
> write to 0xffff88810285ef52 of 1 bytes by interrupt on cpu 0:
>  vring_interrupt+0x12b/0x180 drivers/virtio/virtio_ring.c:2591


Yes, it's racy!

2589:        /* Just a hint for performance: so it's ok that this can be racy! */
2590:        if (vq->event)
2591:            vq->event_triggered = true;


Question: is there a way to annotate code to tell syzbot it's ok?


>  __handle_irq_event_percpu+0x95/0x490 kernel/irq/handle.c:158
>  handle_irq_event_percpu kernel/irq/handle.c:193 [inline]
>  handle_irq_event+0x64/0xf0 kernel/irq/handle.c:210
>  handle_edge_irq+0x16d/0x5b0 kernel/irq/chip.c:831
>  generic_handle_irq_desc include/linux/irqdesc.h:173 [inline]
>  handle_irq arch/x86/kernel/irq.c:247 [inline]
>  call_irq_handler arch/x86/kernel/irq.c:259 [inline]
>  __common_interrupt+0x58/0xe0 arch/x86/kernel/irq.c:285
>  common_interrupt+0x7c/0x90 arch/x86/kernel/irq.c:278
>  asm_common_interrupt+0x26/0x40 arch/x86/include/asm/idtentry.h:693
>  __preempt_count_dec_and_test arch/x86/include/asm/preempt.h:94 [inline]
>  __raw_spin_unlock_irq include/linux/spinlock_api_smp.h:160 [inline]
>  _raw_spin_unlock_irq+0x2f/0x50 kernel/locking/spinlock.c:202
>  process_one_work kernel/workqueue.c:3200 [inline]
>  process_scheduled_works+0x43b/0x9a0 kernel/workqueue.c:3312
>  worker_thread+0x526/0x6e0 kernel/workqueue.c:3389
>  kthread+0x1d1/0x210 kernel/kthread.c:389
>  ret_from_fork+0x4b/0x60 arch/x86/kernel/process.c:147
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
> 
> read to 0xffff88810285ef52 of 1 bytes by interrupt on cpu 1:
>  virtqueue_disable_cb_split drivers/virtio/virtio_ring.c:892 [inline]
>  virtqueue_disable_cb+0x92/0x180 drivers/virtio/virtio_ring.c:2446
>  start_xmit+0x14b/0x1260 drivers/net/virtio_net.c:3034
>  __netdev_start_xmit include/linux/netdevice.h:4913 [inline]
>  netdev_start_xmit include/linux/netdevice.h:4922 [inline]
>  xmit_one net/core/dev.c:3580 [inline]
>  dev_hard_start_xmit+0x119/0x3f0 net/core/dev.c:3596
>  sch_direct_xmit+0x1b0/0x580 net/sched/sch_generic.c:343
>  __dev_xmit_skb net/core/dev.c:3809 [inline]
>  __dev_queue_xmit+0xf3d/0x1fe0 net/core/dev.c:4389
>  dev_queue_xmit include/linux/netdevice.h:3105 [inline]
>  neigh_hh_output include/net/neighbour.h:526 [inline]
>  neigh_output include/net/neighbour.h:540 [inline]
>  ip_finish_output2+0x73d/0x8b0 net/ipv4/ip_output.c:235
>  ip_finish_output+0x11a/0x2a0 net/ipv4/ip_output.c:323
>  NF_HOOK_COND include/linux/netfilter.h:303 [inline]
>  ip_output+0xab/0x170 net/ipv4/ip_output.c:433
>  dst_output include/net/dst.h:450 [inline]
>  ip_local_out net/ipv4/ip_output.c:129 [inline]
>  __ip_queue_xmit+0xbcc/0xbf0 net/ipv4/ip_output.c:535
>  ip_queue_xmit+0x38/0x50 net/ipv4/ip_output.c:549
>  __tcp_transmit_skb+0x15b0/0x1970 net/ipv4/tcp_output.c:1466
>  tcp_transmit_skb net/ipv4/tcp_output.c:1484 [inline]
>  tcp_write_xmit+0x1273/0x31c0 net/ipv4/tcp_output.c:2829
>  __tcp_push_pending_frames+0x6a/0x1a0 net/ipv4/tcp_output.c:3014
>  tcp_push_pending_frames include/net/tcp.h:2106 [inline]
>  tcp_data_snd_check net/ipv4/tcp_input.c:5707 [inline]
>  tcp_rcv_established+0xa12/0xef0 net/ipv4/tcp_input.c:6141
>  tcp_v4_do_rcv+0x662/0x740 net/ipv4/tcp_ipv4.c:1911
>  tcp_v4_rcv+0x1b25/0x1ee0 net/ipv4/tcp_ipv4.c:2346
>  ip_protocol_deliver_rcu+0x370/0x720 net/ipv4/ip_input.c:205
>  ip_local_deliver_finish+0x17d/0x210 net/ipv4/ip_input.c:233
>  NF_HOOK include/linux/netfilter.h:314 [inline]
>  ip_local_deliver+0xec/0x1d0 net/ipv4/ip_input.c:254
>  dst_input include/net/dst.h:460 [inline]
>  ip_sublist_rcv_finish net/ipv4/ip_input.c:580 [inline]
>  ip_list_rcv_finish net/ipv4/ip_input.c:631 [inline]
>  ip_sublist_rcv+0x55a/0x640 net/ipv4/ip_input.c:639
>  ip_list_rcv+0x25e/0x290 net/ipv4/ip_input.c:674
>  __netif_receive_skb_list_ptype net/core/dev.c:5704 [inline]
>  __netif_receive_skb_list_core+0x4f0/0x520 net/core/dev.c:5752
>  __netif_receive_skb_list net/core/dev.c:5804 [inline]
>  netif_receive_skb_list_internal+0x4e4/0x660 net/core/dev.c:5896
>  gro_normal_list include/net/gro.h:515 [inline]
>  napi_complete_done+0x1cb/0x450 net/core/dev.c:6247
>  virtqueue_napi_complete drivers/net/virtio_net.c:694 [inline]
>  virtnet_poll+0x1b73/0x1fd0 drivers/net/virtio_net.c:2826
>  __napi_poll+0x63/0x3c0 net/core/dev.c:6772
>  napi_poll net/core/dev.c:6841 [inline]
>  net_rx_action+0x3a1/0x7f0 net/core/dev.c:6963
>  handle_softirqs+0xc3/0x280 kernel/softirq.c:554
>  __do_softirq kernel/softirq.c:588 [inline]
>  invoke_softirq kernel/softirq.c:428 [inline]
>  __irq_exit_rcu kernel/softirq.c:637 [inline]
>  irq_exit_rcu+0x3e/0x90 kernel/softirq.c:649
>  instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1043 [inline]
>  sysvec_apic_timer_interrupt+0x37/0x80 arch/x86/kernel/apic/apic.c:1043
>  asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:702
> 
> value changed: 0x00 -> 0x01
> 
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 1 UID: 0 PID: 31238 Comm: syz.2.3839 Not tainted 6.11.0-rc7-syzkaller-00021-g7c6a3a65ace7 #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/06/2024
> ==================================================================
> 
> 
> ---
> This report is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
> syzbot engineers can be reached at syzkaller@googlegroups.com.
> 
> syzbot will keep track of this issue. See:
> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> 
> If the report is already addressed, let syzbot know by replying with:
> #syz fix: exact-commit-title
> 
> If you want to overwrite report's subsystems, reply with:
> #syz set subsystems: new-subsystem
> (See the list of subsystem names on the web dashboard)
> 
> If the report is a duplicate of another one, reply with:
> #syz dup: exact-subject-of-another-report
> 
> If you want to undo deduplication, reply with:
> #syz undup

Re: [syzbot] [virt?] KCSAN: data-race in virtqueue_disable_cb / vring_interrupt (4)

[Marco Elver]

On Thu, 12 Sept 2024 at 13:03, Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Thu, Sep 12, 2024 at 01:11:21AM -0700, syzbot wrote:
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    7c6a3a65ace7 minmax: reduce min/max macro expansion in ato..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=1608e49f980000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=1e7d02549be622b2
> > dashboard link: https://syzkaller.appspot.com/bug?extid=8a02104389c2e0ef5049
> > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> >
> > Unfortunately, I don't have any reproducer for this issue yet.
> >
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/a1f7496fa21f/disk-7c6a3a65.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/f423739e51a9/vmlinux-7c6a3a65.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/b65a0f38cbd7/bzImage-7c6a3a65.xz
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+8a02104389c2e0ef5049@syzkaller.appspotmail.com
> >
> > ==================================================================
> > BUG: KCSAN: data-race in virtqueue_disable_cb / vring_interrupt
> >
> > write to 0xffff88810285ef52 of 1 bytes by interrupt on cpu 0:
> >  vring_interrupt+0x12b/0x180 drivers/virtio/virtio_ring.c:2591
>
>
> Yes, it's racy!
>
> 2589:        /* Just a hint for performance: so it's ok that this can be racy! */
> 2590:        if (vq->event)
> 2591:            vq->event_triggered = true;
>
>
> Question: is there a way to annotate code to tell syzbot it's ok?

In this case, "if (data_race(vq->event))" might be the right choice.

This is a quick guide on which access primitive to use in concurrent
code: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/access-marking.txt

Thanks,
-- Marco

Re: [syzbot] [virt?] KCSAN: data-race in virtqueue_disable_cb / vring_interrupt (4)

[Michael S. Tsirkin]

On Thu, Sep 12, 2024 at 03:48:32PM +0200, Marco Elver wrote:
> On Thu, 12 Sept 2024 at 13:03, Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Thu, Sep 12, 2024 at 01:11:21AM -0700, syzbot wrote:
> > > Hello,
> > >
> > > syzbot found the following issue on:
> > >
> > > HEAD commit:    7c6a3a65ace7 minmax: reduce min/max macro expansion in ato..
> > > git tree:       upstream
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=1608e49f980000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=1e7d02549be622b2
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=8a02104389c2e0ef5049
> > > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> > >
> > > Unfortunately, I don't have any reproducer for this issue yet.
> > >
> > > Downloadable assets:
> > > disk image: https://storage.googleapis.com/syzbot-assets/a1f7496fa21f/disk-7c6a3a65.raw.xz
> > > vmlinux: https://storage.googleapis.com/syzbot-assets/f423739e51a9/vmlinux-7c6a3a65.xz
> > > kernel image: https://storage.googleapis.com/syzbot-assets/b65a0f38cbd7/bzImage-7c6a3a65.xz
> > >
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+8a02104389c2e0ef5049@syzkaller.appspotmail.com
> > >
> > > ==================================================================
> > > BUG: KCSAN: data-race in virtqueue_disable_cb / vring_interrupt
> > >
> > > write to 0xffff88810285ef52 of 1 bytes by interrupt on cpu 0:
> > >  vring_interrupt+0x12b/0x180 drivers/virtio/virtio_ring.c:2591
> >
> >
> > Yes, it's racy!
> >
> > 2589:        /* Just a hint for performance: so it's ok that this can be racy! */
> > 2590:        if (vq->event)
> > 2591:            vq->event_triggered = true;
> >
> >
> > Question: is there a way to annotate code to tell syzbot it's ok?
> 
> In this case, "if (data_race(vq->event))" might be the right choice.

No, vq->event is not racy.

The race is between a write and a read of event_triggered.
I think data_race tags a read, it can not tag a write, correct?



> This is a quick guide on which access primitive to use in concurrent
> code: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/access-marking.txt
> 
> Thanks,
> -- Marco

Re: [syzbot] [virt?] KCSAN: data-race in virtqueue_disable_cb / vring_interrupt (4)

[Marco Elver]

On Thu, 12 Sept 2024 at 16:34, Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Thu, Sep 12, 2024 at 03:48:32PM +0200, Marco Elver wrote:
> > On Thu, 12 Sept 2024 at 13:03, Michael S. Tsirkin <mst@redhat.com> wrote:
> > >
> > > On Thu, Sep 12, 2024 at 01:11:21AM -0700, syzbot wrote:
> > > > Hello,
> > > >
> > > > syzbot found the following issue on:
> > > >
> > > > HEAD commit:    7c6a3a65ace7 minmax: reduce min/max macro expansion in ato..
> > > > git tree:       upstream
> > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1608e49f980000
> > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=1e7d02549be622b2
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=8a02104389c2e0ef5049
> > > > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> > > >
> > > > Unfortunately, I don't have any reproducer for this issue yet.
> > > >
> > > > Downloadable assets:
> > > > disk image: https://storage.googleapis.com/syzbot-assets/a1f7496fa21f/disk-7c6a3a65.raw.xz
> > > > vmlinux: https://storage.googleapis.com/syzbot-assets/f423739e51a9/vmlinux-7c6a3a65.xz
> > > > kernel image: https://storage.googleapis.com/syzbot-assets/b65a0f38cbd7/bzImage-7c6a3a65.xz
> > > >
> > > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > > Reported-by: syzbot+8a02104389c2e0ef5049@syzkaller.appspotmail.com
> > > >
> > > > ==================================================================
> > > > BUG: KCSAN: data-race in virtqueue_disable_cb / vring_interrupt
> > > >
> > > > write to 0xffff88810285ef52 of 1 bytes by interrupt on cpu 0:
> > > >  vring_interrupt+0x12b/0x180 drivers/virtio/virtio_ring.c:2591
> > >
> > >
> > > Yes, it's racy!
> > >
> > > 2589:        /* Just a hint for performance: so it's ok that this can be racy! */
> > > 2590:        if (vq->event)
> > > 2591:            vq->event_triggered = true;
> > >
> > >
> > > Question: is there a way to annotate code to tell syzbot it's ok?
> >
> > In this case, "if (data_race(vq->event))" might be the right choice.
>
> No, vq->event is not racy.

Oops - yes.

> The race is between a write and a read of event_triggered.
> I think data_race tags a read, it can not tag a write, correct?

data_race() takes an expression, so either read or write can be
enclosed - e.g. "data_race(vq->event_triggered = true);" works as
well.

Re: [syzbot] [virt?] KCSAN: data-race in virtqueue_disable_cb / vring_interrupt (4)

[Michael S. Tsirkin]

On Thu, Sep 12, 2024 at 04:46:28PM +0200, Marco Elver wrote:
> On Thu, 12 Sept 2024 at 16:34, Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Thu, Sep 12, 2024 at 03:48:32PM +0200, Marco Elver wrote:
> > > On Thu, 12 Sept 2024 at 13:03, Michael S. Tsirkin <mst@redhat.com> wrote:
> > > >
> > > > On Thu, Sep 12, 2024 at 01:11:21AM -0700, syzbot wrote:
> > > > > Hello,
> > > > >
> > > > > syzbot found the following issue on:
> > > > >
> > > > > HEAD commit:    7c6a3a65ace7 minmax: reduce min/max macro expansion in ato..
> > > > > git tree:       upstream
> > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1608e49f980000
> > > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=1e7d02549be622b2
> > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=8a02104389c2e0ef5049
> > > > > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> > > > >
> > > > > Unfortunately, I don't have any reproducer for this issue yet.
> > > > >
> > > > > Downloadable assets:
> > > > > disk image: https://storage.googleapis.com/syzbot-assets/a1f7496fa21f/disk-7c6a3a65.raw.xz
> > > > > vmlinux: https://storage.googleapis.com/syzbot-assets/f423739e51a9/vmlinux-7c6a3a65.xz
> > > > > kernel image: https://storage.googleapis.com/syzbot-assets/b65a0f38cbd7/bzImage-7c6a3a65.xz
> > > > >
> > > > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > > > Reported-by: syzbot+8a02104389c2e0ef5049@syzkaller.appspotmail.com
> > > > >
> > > > > ==================================================================
> > > > > BUG: KCSAN: data-race in virtqueue_disable_cb / vring_interrupt
> > > > >
> > > > > write to 0xffff88810285ef52 of 1 bytes by interrupt on cpu 0:
> > > > >  vring_interrupt+0x12b/0x180 drivers/virtio/virtio_ring.c:2591
> > > >
> > > >
> > > > Yes, it's racy!
> > > >
> > > > 2589:        /* Just a hint for performance: so it's ok that this can be racy! */
> > > > 2590:        if (vq->event)
> > > > 2591:            vq->event_triggered = true;
> > > >
> > > >
> > > > Question: is there a way to annotate code to tell syzbot it's ok?
> > >
> > > In this case, "if (data_race(vq->event))" might be the right choice.
> >
> > No, vq->event is not racy.
> 
> Oops - yes.
> 
> > The race is between a write and a read of event_triggered.
> > I think data_race tags a read, it can not tag a write, correct?
> 
> data_race() takes an expression, so either read or write can be
> enclosed - e.g. "data_race(vq->event_triggered = true);" works as
> well.

Ah, good to know. Yes, I see it is basically __kcsan_disable_current +
__kcsan_enable_current.  Will do.



-- 
MST

Re: [syzbot] [virt?] KCSAN: data-race in virtqueue_disable_cb / vring_interrupt (4)

[Michael S. Tsirkin]

On Thu, Sep 12, 2024 at 03:48:32PM +0200, Marco Elver wrote:
> On Thu, 12 Sept 2024 at 13:03, Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Thu, Sep 12, 2024 at 01:11:21AM -0700, syzbot wrote:
> > > Hello,
> > >
> > > syzbot found the following issue on:
> > >
> > > HEAD commit:    7c6a3a65ace7 minmax: reduce min/max macro expansion in ato..
> > > git tree:       upstream
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=1608e49f980000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=1e7d02549be622b2
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=8a02104389c2e0ef5049
> > > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> > >
> > > Unfortunately, I don't have any reproducer for this issue yet.
> > >
> > > Downloadable assets:
> > > disk image: https://storage.googleapis.com/syzbot-assets/a1f7496fa21f/disk-7c6a3a65.raw.xz
> > > vmlinux: https://storage.googleapis.com/syzbot-assets/f423739e51a9/vmlinux-7c6a3a65.xz
> > > kernel image: https://storage.googleapis.com/syzbot-assets/b65a0f38cbd7/bzImage-7c6a3a65.xz
> > >
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+8a02104389c2e0ef5049@syzkaller.appspotmail.com
> > >
> > > ==================================================================
> > > BUG: KCSAN: data-race in virtqueue_disable_cb / vring_interrupt
> > >
> > > write to 0xffff88810285ef52 of 1 bytes by interrupt on cpu 0:
> > >  vring_interrupt+0x12b/0x180 drivers/virtio/virtio_ring.c:2591
> >
> >
> > Yes, it's racy!
> >
> > 2589:        /* Just a hint for performance: so it's ok that this can be racy! */
> > 2590:        if (vq->event)
> > 2591:            vq->event_triggered = true;
> >
> >
> > Question: is there a way to annotate code to tell syzbot it's ok?
> 
> In this case, "if (data_race(vq->event))" might be the right choice.
> 
> This is a quick guide on which access primitive to use in concurrent
> code: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/access-marking.txt
> 
> Thanks,
> -- Marco

I sent a patch tagging this whole field as racy. Pls take a look.

-- 
MST