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

[Sean Christopherson <seanjc@google.com>]

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>