[syzbot ci] Re: KVM: x86/hyperv: Fix racy usage of vcpu->arch.hyperv

[syzbot ci]

syzbot ci has tested the following series

[v1] KVM: x86/hyperv: Fix racy usage of vcpu->arch.hyperv
https://lore.kernel.org/all/20260423140833....@google.com
* [PATCH 1/5] KVM: x86/hyperv: Get target FIFO in hv_tlb_flush_enqueue(), not caller
* [PATCH 2/5] KVM: x86/hyperv: Check for NULL vCPU Hyper-V object in kvm_hv_get_tlb_flush_fifo()
* [PATCH 3/5] KVM: x86/hyperv: Ensure vCPU's Hyper-V object is initialized on cross-vCPU accesses
* [PATCH 4/5] KVM: x86/hyperv: Assert vCPU's mutex is held in to_hv_vcpu()
* [PATCH 5/5] 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/e1f68dae-f32a-4112-a163-7a00f36d6508

***

WARNING in kvm_hv_vcpu_uninit

tree: linux-next
URL: https://kernel.googlesource.com/pub/scm/linux/kernel/git/next/linux-next
base: 85f871f6ba46f20d7fbc0b016b4db648c33220dd
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/4f8c36c7-e865-4f65-bb81-e22962e8a1e0/config
syz repro: https://ci.syzbot.org/findings/60fac5d8-1296-4ec7-a8e3-40f561d007e9/syz_repro

------------[ cut here ]------------
debug_locks && !(lock_is_held(&(&vcpu->mutex)->dep_map) || !refcount_read(&vcpu->kvm->users_count))
WARNING: arch/x86/kvm/hyperv.h:79 at to_hv_vcpu arch/x86/kvm/hyperv.h:78 [inline], CPU#1: syz.2.19/5974
WARNING: arch/x86/kvm/hyperv.h:79 at kvm_hv_vcpu_uninit+0x163/0x1b0 arch/x86/kvm/hyperv.c:906, CPU#1: syz.2.19/5974
Modules linked in:
CPU: 1 UID: 0 PID: 5974 Comm: syz.2.19 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:to_hv_vcpu arch/x86/kvm/hyperv.h:78 [inline]
RIP: 0010:kvm_hv_vcpu_uninit+0x163/0x1b0 arch/x86/kvm/hyperv.c:906
Code: 48 89 df e8 0f 0f d8 00 48 c7 03 00 00 00 00 eb 05 e8 e1 8c 6d 00 5b 41 5c 41 5e 41 5f 5d e9 c4 a8 5a 0a cc e8 ce 8c 6d 00 90 <0f> 0b 90 e9 65 ff ff ff 48 c7 c1 00 12 12 90 80 e1 07 80 c1 03 38
RSP: 0018:ffffc90003e07940 EFLAGS: 00010293
RAX: ffffffff81592fe2 RBX: ffff8881b6acd380 RCX: ffff8881046e4980
RDX: 0000000000000000 RSI: 0000000000000002 RDI: 0000000000000000
RBP: 0000000000000002 R08: ffff88817696d743 R09: 1ffff1102ed2dae8
R10: dffffc0000000000 R11: ffffed102ed2dae9 R12: 0000000000000000
R13: 00000000fffffff8 R14: ffff88817696d740 R15: dffffc0000000000
FS: 00007f4cc3a9b6c0(0000) GS:ffff8882a944f000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f4cc2c4edd5 CR3: 000000016d918000 CR4: 0000000000352ef0
Call Trace:
<TASK>
kvm_arch_vcpu_destroy+0x1a9/0x380 arch/x86/kvm/x86.c:12963
kvm_vm_ioctl_create_vcpu+0x69a/0x930 virt/kvm/kvm_main.c:4269
kvm_vm_ioctl+0x893/0xd50 virt/kvm/kvm_main.c:5168
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+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f4cc2b9c819
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:00007f4cc3a9b028 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f4cc2e15fa0 RCX: 00007f4cc2b9c819
RDX: 0000000000000000 RSI: 000000000000ae41 RDI: 0000000000000004
RBP: 00007f4cc2c32c91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f4cc2e16038 R14: 00007f4cc2e15fa0 R15: 00007ffd36efc848
</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: syz...@syzkaller.appspotmail.com

---
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[Sean Christopherson]

On Thu, Apr 23, 2026, syzbot ci wrote:
> syzbot ci has tested the following series

> ***
>
> WARNING in kvm_hv_vcpu_uninit
>
> tree: linux-next
> URL: https://kernel.googlesource.com/pub/scm/linux/kernel/git/next/linux-next
> base: 85f871f6ba46f20d7fbc0b016b4db648c33220dd
> 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/4f8c36c7-e865-4f65-bb81-e22962e8a1e0/config
> syz repro: https://ci.syzbot.org/findings/60fac5d8-1296-4ec7-a8e3-40f561d007e9/syz_repro
>
> ------------[ cut here ]------------
> debug_locks && !(lock_is_held(&(&vcpu->mutex)->dep_map) || !refcount_read(&vcpu->kvm->users_count))
> WARNING: arch/x86/kvm/hyperv.h:79 at to_hv_vcpu arch/x86/kvm/hyperv.h:78 [inline], CPU#1: syz.2.19/5974
> WARNING: arch/x86/kvm/hyperv.h:79 at kvm_hv_vcpu_uninit+0x163/0x1b0 arch/x86/kvm/hyperv.c:906, CPU#1: syz.2.19/5974
> Modules linked in:
> CPU: 1 UID: 0 PID: 5974 Comm: syz.2.19 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:to_hv_vcpu arch/x86/kvm/hyperv.h:78 [inline]
> RIP: 0010:kvm_hv_vcpu_uninit+0x163/0x1b0 arch/x86/kvm/hyperv.c:906

> Call Trace:
> <TASK>
> kvm_arch_vcpu_destroy+0x1a9/0x380 arch/x86/kvm/x86.c:12963
> kvm_vm_ioctl_create_vcpu+0x69a/0x930 virt/kvm/kvm_main.c:4269
> kvm_vm_ioctl+0x893/0xd50 virt/kvm/kvm_main.c:5168
> 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+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
> entry_SYSCALL_64_after_hwframe+0x77/0x7f

Argh, what a pain. It's effectively the same issue that I fudged around in
kvm_hv_set_cpuid(): KVM queries HyperV state during vCPU creation, before taking
vcpu->mutex makes any sense.

One thought I had was to initialize vcpu_idx to -1, so that to_hv_vcpu() could
detect that the vCPU isn't yet visible to others. Arguably that would be also
nice-to-have as it would harden against consuming vcpu->vcpu_idx before it's
fully initialized. As-is, goofs would result in KVM thinking its vCPU0.

diff --git virt/kvm/kvm_main.c virt/kvm/kvm_main.c
index 7fcb92c69dc8..35e92cfb2a45 100644
--- virt/kvm/kvm_main.c
+++ virt/kvm/kvm_main.c
@@ -4198,6 +4198,8 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
goto vcpu_decrement;
}

+ vcpu->vcpu_idx = -1;
+
BUILD_BUG_ON(sizeof(struct kvm_run) > PAGE_SIZE);
page = alloc_page(GFP_KERNEL_ACCOUNT | __GFP_ZERO);
if (!page) {