Subject: Re: [syzbot] [virt?] KCSAN: data-race in virtqueue_disable_cb / virtqueue_enable_cb_delayed (11)

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 12 Apr 2026 20:33:17 -0400

On Mon, Apr 06, 2026 at 11:03:26AM -0600, Venkatesh Srinivas wrote:
> On Tue, Mar 31, 2026 at 8:51 AM Michael S. Tsirkin <[email-masked]> wrote:
> >
> > On Tue, Mar 31, 2026 at 05:18:24AM -0700, syzbot wrote:
> > > Hello,
> > >
> > > syzbot found the following issue on:
> > >
> > > HEAD commit:    46b513250491 Merge tag 'v7.0-rc5-smb3-client-fix' of git:/..
> > > git tree:       upstream
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=1226df72580000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=3a78dd265deac3a9
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=574895e85c21fa090ff6
> > > compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
> > >
> > > Unfortunately, I don't have any reproducer for this issue yet.
> > >
> > > Downloadable assets:
> > > disk image: https://storage.googleapis.com/syzbot-assets/8f19c67785a8/disk-46b51325.raw.xz
> > > vmlinux: https://storage.googleapis.com/syzbot-assets/11dbb9704e20/vmlinux-46b51325.xz
> > > kernel image: https://storage.googleapis.com/syzbot-assets/190d9812e855/bzImage-46b51325.xz
> > >
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: [email-masked]
> > >
> > > ==================================================================
> > > BUG: KCSAN: data-race in virtqueue_disable_cb / virtqueue_enable_cb_delayed
> > >
> > > write to 0xffff8881027a9588 of 2 bytes by interrupt on cpu 1:
> > >  virtqueue_enable_cb_delayed_split drivers/virtio/virtio_ring.c:1102 [inline]
> > >  virtqueue_enable_cb_delayed+0x20f/0x660 drivers/virtio/virtio_ring.c:3196
> > >  start_xmit+0x15ef/0x1ab0 drivers/net/virtio_net.c:3377
> > >  __netdev_start_xmit include/linux/netdevice.h:5325 [inline]
> > >  netdev_start_xmit include/linux/netdevice.h:5334 [inline]
> > >  xmit_one net/core/dev.c:3883 [inline]
> > >  dev_hard_start_xmit+0x136/0x3f0 net/core/dev.c:3899
> > >  sch_direct_xmit+0x192/0x550 net/sched/sch_generic.c:347
> > >  __dev_xmit_skb net/core/dev.c:4198 [inline]
> > >  __dev_queue_xmit+0xca9/0x1f20 net/core/dev.c:4814
> > >  dev_queue_xmit include/linux/netdevice.h:3385 [inline]
> > >  neigh_hh_output include/net/neighbour.h:540 [inline]
> > >  neigh_output include/net/neighbour.h:554 [inline]
> > >  ip_finish_output2+0x705/0x8c0 net/ipv4/ip_output.c:237
> > >  __ip_finish_output net/ipv4/ip_output.c:-1 [inline]
> > >  ip_finish_output+0x112/0x290 net/ipv4/ip_output.c:325
> > >  NF_HOOK_COND include/linux/netfilter.h:307 [inline]
> > >  ip_output+0xbd/0x1c0 net/ipv4/ip_output.c:438
> > >  dst_output include/net/dst.h:470 [inline]
> > >  ip_local_out net/ipv4/ip_output.c:131 [inline]
> > >  __ip_queue_xmit+0xb68/0xba0 net/ipv4/ip_output.c:534
> > >  ip_queue_xmit+0x39/0x50 net/ipv4/ip_output.c:548
> > >  __tcp_transmit_skb+0x1af2/0x1f10 net/ipv4/tcp_output.c:1693
> > >  tcp_transmit_skb net/ipv4/tcp_output.c:1711 [inline]
> > >  tcp_write_xmit+0x1597/0x3640 net/ipv4/tcp_output.c:3064
> > >  __tcp_push_pending_frames+0x6d/0x1b0 net/ipv4/tcp_output.c:3247
> > >  tcp_push_pending_frames include/net/tcp.h:2285 [inline]
> > >  tcp_data_snd_check net/ipv4/tcp_input.c:6127 [inline]
> > >  tcp_rcv_established+0xda2/0x12f0 net/ipv4/tcp_input.c:6610
> > >  tcp_v4_do_rcv+0x91d/0xa30 net/ipv4/tcp_ipv4.c:1884
> > >  tcp_v4_rcv+0x19f8/0x1db0 net/ipv4/tcp_ipv4.c:2319
> > >  ip_protocol_deliver_rcu+0x395/0x790 net/ipv4/ip_input.c:207
> > >  ip_local_deliver_finish+0x1fc/0x2f0 net/ipv4/ip_input.c:241
> > >  NF_HOOK include/linux/netfilter.h:318 [inline]
> > >  ip_local_deliver+0xe8/0x1e0 net/ipv4/ip_input.c:262
> > >  dst_input include/net/dst.h:480 [inline]
> > >  ip_sublist_rcv_finish net/ipv4/ip_input.c:584 [inline]
> > >  ip_list_rcv_finish net/ipv4/ip_input.c:636 [inline]
> > >  ip_sublist_rcv+0x5a4/0x6a0 net/ipv4/ip_input.c:644
> > >  ip_list_rcv+0x261/0x290 net/ipv4/ip_input.c:678
> > >  __netif_receive_skb_list_ptype net/core/dev.c:6219 [inline]
> > >  __netif_receive_skb_list_core+0x4dc/0x500 net/core/dev.c:6266
> > >  __netif_receive_skb_list net/core/dev.c:6318 [inline]
> > >  netif_receive_skb_list_internal+0x47d/0x5f0 net/core/dev.c:6409
> > >  gro_normal_list include/net/gro.h:523 [inline]
> > >  gro_flush_normal include/net/gro.h:531 [inline]
> > >  napi_complete_done+0x19c/0x3f0 net/core/dev.c:6777
> > >  virtqueue_napi_complete drivers/net/virtio_net.c:749 [inline]
> > >  virtnet_poll+0x1bb1/0x2040 drivers/net/virtio_net.c:3091
> > >  __napi_poll+0x61/0x330 net/core/dev.c:7704
> > >  napi_poll net/core/dev.c:7767 [inline]
> > >  net_rx_action+0x452/0x930 net/core/dev.c:7924
> > >  handle_softirqs+0xb9/0x2a0 kernel/softirq.c:622
> > >  __do_softirq kernel/softirq.c:656 [inline]
> > >  invoke_softirq kernel/softirq.c:496 [inline]
> > >  __irq_exit_rcu+0x39/0xc0 kernel/softirq.c:723
> > >  common_interrupt+0x83/0x90 arch/x86/kernel/irq.c:326
> > >  asm_common_interrupt+0x26/0x40 arch/x86/include/asm/idtentry.h:688
> > >  finish_task_switch+0x86/0x280 kernel/sched/core.c:5155
> > >  context_switch kernel/sched/core.c:5301 [inline]
> > >  __schedule+0x93c/0xd40 kernel/sched/core.c:6911
> > >  __schedule_loop kernel/sched/core.c:6993 [inline]
> > >  schedule+0x5e/0xd0 kernel/sched/core.c:7008
> > >  schedule_timeout+0xca/0x180 kernel/time/sleep_timeout.c:99
> > >  io_wq_worker+0x3a0/0x970 io_uring/io-wq.c:728
> > >  ret_from_fork+0x150/0x360 arch/x86/kernel/process.c:158
> > >  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
> > >
> > > read to 0xffff8881027a9588 of 2 bytes by interrupt on cpu 0:
> > >  virtqueue_disable_cb_split drivers/virtio/virtio_ring.c:1046 [inline]
> > >  virtqueue_disable_cb+0x4c/0x2c0 drivers/virtio/virtio_ring.c:3108
> > >  virtqueue_napi_schedule drivers/net/virtio_net.c:738 [inline]
> > >  skb_xmit_done+0xb0/0x1a0 drivers/net/virtio_net.c:786
> > >  vring_interrupt+0x2d7/0x310 drivers/virtio/virtio_ring.c:3254
> > >  __handle_irq_event_percpu+0x9c/0x4d0 kernel/irq/handle.c:209
> > >  handle_irq_event_percpu kernel/irq/handle.c:246 [inline]
> > >  handle_irq_event+0x64/0xf0 kernel/irq/handle.c:263
> > >  handle_edge_irq+0x154/0x470 kernel/irq/chip.c:855
> > >  generic_handle_irq_desc include/linux/irqdesc.h:186 [inline]
> > >  handle_irq arch/x86/kernel/irq.c:262 [inline]
> > >  call_irq_handler arch/x86/kernel/irq.c:-1 [inline]
> > >  __common_interrupt+0x60/0xb0 arch/x86/kernel/irq.c:333
> > >  common_interrupt+0x7e/0x90 arch/x86/kernel/irq.c:326
> > >  asm_common_interrupt+0x26/0x40 arch/x86/include/asm/idtentry.h:688
> > >  decode_watchpoint kernel/kcsan/encoding.h:74 [inline]
> > >  find_watchpoint kernel/kcsan/core.c:132 [inline]
> > >  check_access kernel/kcsan/core.c:737 [inline]
> > >  __tsan_read8+0x31/0x190 kernel/kcsan/core.c:1025
> > >  _find_next_bit+0x29/0x90 lib/find_bit.c:157
> > >  find_next_bit include/linux/find.h:73 [inline]
> > >  ebitmap_next_positive security/selinux/ss/ebitmap.h:72 [inline]
> > >  context_struct_compute_av+0x496/0xaf0 security/selinux/ss/services.c:661
> > >  security_compute_av+0x34f/0xa20 security/selinux/ss/services.c:1177
> > >  avc_compute_av+0x5d/0x430 security/selinux/avc.c:992
> > >  avc_perm_nonode+0x5e/0xe0 security/selinux/avc.c:1117
> > >  avc_has_perm_noaudit+0xf2/0x130 security/selinux/avc.c:1160
> > >  avc_has_perm+0x60/0x190 security/selinux/avc.c:1195
> > >  inode_has_perm security/selinux/hooks.c:1691 [inline]
> > >  file_has_perm security/selinux/hooks.c:1787 [inline]
> > >  selinux_revalidate_file_permission security/selinux/hooks.c:3793 [inline]
> > >  selinux_file_permission+0x633/0x690 security/selinux/hooks.c:3814
> > >  security_file_permission+0x3a/0x70 security/security.c:2367
> > >  rw_verify_area fs/read_write.c:475 [inline]
> > >  vfs_write+0x135/0x9f0 fs/read_write.c:679
> > >  ksys_write+0xdc/0x1a0 fs/read_write.c:740
> > >  __do_sys_write fs/read_write.c:751 [inline]
> > >  __se_sys_write fs/read_write.c:748 [inline]
> > >  __x64_sys_write+0x40/0x50 fs/read_write.c:748
> > >  x64_sys_call+0x27e1/0x3020 arch/x86/include/generated/asm/syscalls_64.h:2
> > >  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> > >  do_syscall_64+0x12c/0x370 arch/x86/entry/syscall_64.c:94
> > >  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> > >
> > > value changed: 0x0001 -> 0x0000
> > >
> > > Reported by Kernel Concurrency Sanitizer on:
> > > CPU: 0 UID: 0 PID: 3302 Comm: syz-executor Tainted: G        W           syzkaller #0 PREEMPT(full)
> > > Tainted: [W]=WARN
> > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
> > > ==================================================================
> > >
> > >
> > > ---
> > > 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 [email-masked].
> > >
> > > 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
> >
> >
> > The issue seems to be that disable_cb writes flags and flags shadow,
> > without locks.
> >
> > so if it sets VRING_AVAIL_F_NO_INTERRUPT in both, it's possible
> > that we have a race:
> >
> > CPU1 is in start_xmit() on sq->vq.
> > It has already done the entry-side virtqueue_disable_cb().
> > Before it reaches the tail-side virtqueue_enable_cb_delayed(), the device completes something and raises an IRQ on CPU0.
> > vring_interrupt() calls skb_xmit_done() on the same sq->vq.
> > That IRQ path calls virtqueue_disable_cb(vq) concurrently with CPU1's virtqueue_enable_cb_delayed(sq->vq).
> >
> > Now:
> >
> >
> > disable cb:
> >         set VRING_AVAIL_F_NO_INTERRUPT in shadow
> >
> >
> >         enable_cb:
> >                 clear VRING_AVAIL_F_NO_INTERRUPT in shadow
> >                 clear VRING_AVAIL_F_NO_INTERRUPT in flags
> >
> >
> >         set VRING_AVAIL_F_NO_INTERRUPT in shadow
> >
> >
> > and now they are out of sync: cleared in shadow (so next enable
> > will be a nop) and set in flags (so we do not get another interrupt).
> >
> >
> > I frankly think the only fix is to drop the flags shadow.
> 
> I'm working on a kcsan-safe fix right now.

any update? need help? I have some ideas if you like ...

> > Venkatesh, do you know how much perf gain did we get from
> > not poking at flags directly?
> 
> On the obsolete h/w described in the original commit,
> 2 - 3% on a microbenchmark. Newer hardware not
> widely available at the time (Skylake-SP) saw similar
> wins. Can revive the microbenchmark and get new
> data.
> 
> The real motivation was alluded to in the commit -- if
> you implement a physical (PCIe) virtio device and put
> the vring in device memory, writes are reasonably
> fast (buffered, posted), but reads were very heavyweight.
> Shadowing the flags field removed this read and made
> the performance of that teneble.
> 
> 
> -- vs;