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 protected]> 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 protected] > > > > > > ================================================================== > > > 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 protected]. > > > > > > 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;
