arch/x86/kvm/hyperv.c | 64 +++++++++++++++++++++------------------ arch/x86/kvm/hyperv.h | 25 +++++++++++++-- arch/x86/kvm/vmx/nested.h | 6 ++-- include/linux/kvm_host.h | 7 +++++ virt/kvm/kvm_main.c | 2 ++ 5 files changed, 67 insertions(+), 37 deletions(-)
Fix a bug found by syzkaller (on a Google-internal kernel) where KVM consumes
a vCPU's HyperV structure before it's fully initialized, by concurrently
triggering PV TLB flushes (queues flushes into a vCPU's FIFO without holding
the vCPU's mutex) on a vCPU that is in the process of activating HyperV.
Harden against similar bugs by asserting the vcpu->mutex is held when using
the "normal" to_hv_vcpu(), same as we did for get_vmcs12() and
get_shadow_vmcs12() (also in response to cross-task races). To avoid false
positives when creating a vCPU, initialize vcpu_idx to -1, and treat the vCPU
as unreachable (other than the caller, obviously) if its index is -1.
v2:
- Init vcpu->vcpu_idx to -1, use that as a canary to detect the vCPU is
unreachable, and allow accessing Hyper-V state if the vCPU is otherwise
unreachable. [syzbot]
v1: https://lore.kernel.org/all/20260423140833.439512-1-seanjc@google.com
Sean Christopherson (8):
KVM: x86/hyperv: Get target FIFO in hv_tlb_flush_enqueue(), not caller
KVM: x86/hyperv: Check for NULL vCPU Hyper-V object in
kvm_hv_get_tlb_flush_fifo()
KVM: x86/hyperv: Ensure vCPU's Hyper-V object is initialized on
cross-vCPU accesses
KVM: Initialize a vCPU's index to '-1' while it's being created
KVM: Move nVMX's lockdep logic for vcpu->mutex to a common helper
KVM: x86: Treat a vCPU as unreachable if its index is invalid
KVM: x86/hyperv: Assert vCPU's mutex is held in to_hv_vcpu()
KVM: x86/hyperv: Use {READ,WRITE}_ONCE for cross-task synic->active
accesses
arch/x86/kvm/hyperv.c | 64 +++++++++++++++++++++------------------
arch/x86/kvm/hyperv.h | 25 +++++++++++++--
arch/x86/kvm/vmx/nested.h | 6 ++--
include/linux/kvm_host.h | 7 +++++
virt/kvm/kvm_main.c | 2 ++
5 files changed, 67 insertions(+), 37 deletions(-)
base-commit: c1f7303302927f9cbf4efedf70f0512cde168c65
--
2.54.0.1136.gdb2ca164c4-goog
syzbot ci has tested the following series
[v2] KVM: x86/hyperv: Fix racy usage of vcpu->arch.hyperv
https://lore.kernel.org/all/20260612230622.687665-1-seanjc@google.com
* [PATCH v2 1/8] KVM: x86/hyperv: Get target FIFO in hv_tlb_flush_enqueue(), not caller
* [PATCH v2 2/8] KVM: x86/hyperv: Check for NULL vCPU Hyper-V object in kvm_hv_get_tlb_flush_fifo()
* [PATCH v2 3/8] KVM: x86/hyperv: Ensure vCPU's Hyper-V object is initialized on cross-vCPU accesses
* [PATCH v2 4/8] KVM: Initialize a vCPU's index to '-1' while it's being created
* [PATCH v2 5/8] KVM: Move nVMX's lockdep logic for vcpu->mutex to a common helper
* [PATCH v2 6/8] KVM: x86: Treat a vCPU as unreachable if its index is invalid
* [PATCH v2 7/8] KVM: x86/hyperv: Assert vCPU's mutex is held in to_hv_vcpu()
* [PATCH v2 8/8] KVM: x86/hyperv: Use {READ,WRITE}_ONCE for cross-task synic->active accesses
and found the following issue:
WARNING in kvm_hv_vcpu_uninit
Full report is available here:
https://ci.syzbot.org/series/674ef35a-9335-4710-8a6d-b18d01510cbb
***
WARNING in kvm_hv_vcpu_uninit
tree: linux-next
URL: https://kernel.googlesource.com/pub/scm/linux/kernel/git/next/linux-next
base: c1f7303302927f9cbf4efedf70f0512cde168c65
arch: amd64
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
config: https://ci.syzbot.org/builds/20dd789e-0a89-4465-844c-9d91b5ce4a45/config
syz repro: https://ci.syzbot.org/findings/eeefcfe6-b8e9-4c5b-900e-855d814f5d97/syz_repro
------------[ cut here ]------------
debug_locks && !(lock_is_held(&(&vcpu->mutex)->dep_map) || vcpu->vcpu_idx < 0 || !refcount_read(&vcpu->kvm->users_count))
WARNING: ./include/linux/kvm_host.h:996 at kvm_lockdep_assert_vcpu_is_locked_or_unreachable include/linux/kvm_host.h:994 [inline], CPU#1: syz.1.25/5883
WARNING: ./include/linux/kvm_host.h:996 at to_hv_vcpu arch/x86/kvm/hyperv.h:78 [inline], CPU#1: syz.1.25/5883
WARNING: ./include/linux/kvm_host.h:996 at kvm_hv_vcpu_uninit+0x198/0x210 arch/x86/kvm/hyperv.c:906, CPU#1: syz.1.25/5883
Modules linked in:
CPU: 1 UID: 0 PID: 5883 Comm: syz.1.25 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
RIP: 0010:kvm_lockdep_assert_vcpu_is_locked_or_unreachable include/linux/kvm_host.h:994 [inline]
RIP: 0010:to_hv_vcpu arch/x86/kvm/hyperv.h:78 [inline]
RIP: 0010:kvm_hv_vcpu_uninit+0x198/0x210 arch/x86/kvm/hyperv.c:906
Code: 48 89 df e8 0a ba d8 00 48 c7 03 00 00 00 00 eb 05 e8 1c 63 6d 00 5b 41 5c 41 5e 41 5f 5d e9 8f f6 51 0a cc e8 09 63 6d 00 90 <0f> 0b 90 e9 65 ff ff ff 48 c7 c1 e0 5c 30 90 80 e1 07 80 c1 03 38
RSP: 0018:ffffc900035ef940 EFLAGS: 00010293
RAX: ffffffff8158f397 RBX: ffff88816987d400 RCX: ffff888110b20000
RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000000
RBP: 0000000000000001 R08: ffff88810c5b9703 R09: 1ffff110218b72e0
R10: dffffc0000000000 R11: ffffed10218b72e1 R12: 0000000000000000
R13: 00000000fffffff8 R14: ffff88810c5b9700 R15: dffffc0000000000
FS: 00007fbc073dc6c0(0000) GS:ffff8882a929a000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f451a40fff8 CR3: 000000010f703000 CR4: 0000000000352ef0
Call Trace:
<TASK>
kvm_arch_vcpu_destroy+0x1a9/0x380 arch/x86/kvm/x86.c:12905
kvm_vm_ioctl_create_vcpu+0x68b/0x940 virt/kvm/kvm_main.c:4262
kvm_vm_ioctl+0x893/0xd50 virt/kvm/kvm_main.c:5161
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fbc0659ce59
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fbc073dc028 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007fbc06815fa0 RCX: 00007fbc0659ce59
RDX: 0000000000000001 RSI: 000000000000ae41 RDI: 00000000000000f7
RBP: 00007fbc06632d6f R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fbc06816038 R14: 00007fbc06815fa0 R15: 00007fffc5de9fc8
</TASK>
***
If these findings have caused you to resend the series or submit a
separate fix, please add the following tag to your commit message:
Tested-by: syzbot@syzkaller.appspotmail.com
---
This report is generated by a bot. It may contain errors.
syzbot ci engineers can be reached at syzkaller@googlegroups.com.
To test a patch for this bug, please reply with `#syz test`
(should be on a separate line).
The patch should be attached to the email.
Note: arguments like custom git repos and branches are not supported.
On Sat, Jun 13, 2026, syzbot ci wrote:
> Full report is available here:
> https://ci.syzbot.org/series/674ef35a-9335-4710-8a6d-b18d01510cbb
>
> ***
>
> WARNING in kvm_hv_vcpu_uninit
>
> tree: linux-next
> URL: https://kernel.googlesource.com/pub/scm/linux/kernel/git/next/linux-next
> base: c1f7303302927f9cbf4efedf70f0512cde168c65
> arch: amd64
> compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
> config: https://ci.syzbot.org/builds/20dd789e-0a89-4465-844c-9d91b5ce4a45/config
> syz repro: https://ci.syzbot.org/findings/eeefcfe6-b8e9-4c5b-900e-855d814f5d97/syz_repro
>
> ------------[ cut here ]------------
> debug_locks && !(lock_is_held(&(&vcpu->mutex)->dep_map) || vcpu->vcpu_idx < 0 || !refcount_read(&vcpu->kvm->users_count))
> WARNING: ./include/linux/kvm_host.h:996 at kvm_lockdep_assert_vcpu_is_locked_or_unreachable include/linux/kvm_host.h:994 [inline], CPU#1: syz.1.25/5883
> WARNING: ./include/linux/kvm_host.h:996 at to_hv_vcpu arch/x86/kvm/hyperv.h:78 [inline], CPU#1: syz.1.25/5883
> WARNING: ./include/linux/kvm_host.h:996 at kvm_hv_vcpu_uninit+0x198/0x210 arch/x86/kvm/hyperv.c:906, CPU#1: syz.1.25/5883
> Modules linked in:
> CPU: 1 UID: 0 PID: 5883 Comm: syz.1.25 Not tainted syzkaller #0 PREEMPT(full)
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
> RIP: 0010:kvm_lockdep_assert_vcpu_is_locked_or_unreachable include/linux/kvm_host.h:994 [inline]
> RIP: 0010:to_hv_vcpu arch/x86/kvm/hyperv.h:78 [inline]
> RIP: 0010:kvm_hv_vcpu_uninit+0x198/0x210 arch/x86/kvm/hyperv.c:906
> Call Trace:
> <TASK>
> kvm_arch_vcpu_destroy+0x1a9/0x380 arch/x86/kvm/x86.c:12905
> kvm_vm_ioctl_create_vcpu+0x68b/0x940 virt/kvm/kvm_main.c:4262
Darn error injection. xa_insert() can fail with -ENOMEM. It's kinda ugly, but
unwinding vcpu_idx on failure is a logically sound fix. And a good excuse to
document that the index needs to be set before adding it to the array, e.g.
diff --git virt/kvm/kvm_main.c virt/kvm/kvm_main.c
index b3d2a678210c..98da4c889ffc 100644
--- virt/kvm/kvm_main.c
+++ virt/kvm/kvm_main.c
@@ -4218,11 +4218,18 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
goto unlock_vcpu_destroy;
}
+ /*
+ * Set the vCPU's index *before* the vCPU is reachable by other tasks.
+ * Unwind the index back to -1 on failure so that KVM can use the index
+ * to detect that the vCPU is unreachable, e.g. for lockdep asserts.
+ */
vcpu->vcpu_idx = atomic_read(&kvm->online_vcpus);
r = xa_insert(&kvm->vcpu_array, vcpu->vcpu_idx, vcpu, GFP_KERNEL_ACCOUNT);
WARN_ON_ONCE(r == -EBUSY);
- if (r)
+ if (r) {
+ vcpu->vcpu_idx = -1;
goto unlock_vcpu_destroy;
+ }
/*
* Now it's all set up, let userspace reach it. Grab the vCPU's mutex
> kvm_vm_ioctl+0x893/0xd50 virt/kvm/kvm_main.c:5161
> vfs_ioctl fs/ioctl.c:51 [inline]
> __do_sys_ioctl fs/ioctl.c:597 [inline]
> __se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
> do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7fbc0659ce59
> Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007fbc073dc028 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> RAX: ffffffffffffffda RBX: 00007fbc06815fa0 RCX: 00007fbc0659ce59
> RDX: 0000000000000001 RSI: 000000000000ae41 RDI: 00000000000000f7
> RBP: 00007fbc06632d6f R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> R13: 00007fbc06816038 R14: 00007fbc06815fa0 R15: 00007fffc5de9fc8
> </TASK>
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