Re: [syzbot] [kvm?] WARNING in vcpu_run

Re: [syzbot] [kvm?] WARNING in vcpu_run

[syzbot]

> #syz test git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git

I've failed to parse your command.
Did you perhaps forget to provide the branch name, or added an extra ':'?
Please use one of the two supported formats:
1. #syz test
2. #syz test: repo branch-or-commit-hash
Note the lack of ':' in option 1.

> master
>
> From 2090baeb71f48c9ad62129bd457a2c3bb5f0ed55 Mon Sep 17 00:00:00 2001
> From: Alessandro Ratti <alessandro@0x65c.net>
> Date: Sat, 3 Jan 2026 22:01:32 +0100
> Subject: [PATCH] KVM: x86: Handle -EBUSY from nested event check in
>  vcpu_block()
>
> When a vCPU running in nested guest mode attempts to block (e.g. due to
> HLT), kvm_check_nested_events() may return -EBUSY to indicate that a
> nested event is pending but cannot be injected immediately, such as
> when event delivery is temporarily blocked in the guest.
>
> Currently, vcpu_block() treats this as a generic error and exits to
> userspace. This can cause the vCPU to repeatedly block without making
> forward progress, delaying nested event injection and potentially
> leading to guest hangs under rare timing conditions.
>
> Handle -EBUSY explicitly by returning to the vCPU run loop and retrying
> guest entry instead of blocking. This allows nested event delivery to
> complete once the temporary blocking condition clears.
>
> This issue was triggered by syzkaller during nested virtualization
> stress testing.
>
> Fixes: 45405155d876 ("KVM: x86: WARN if a vCPU gets a valid wakeup
> that KVM can't yet inject")
> Reported-by: syzbot+1522459a74d26b0ac33a@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?id=b646ef310afe5b51ae0372e1de8fdd68baad9eb5
> Signed-off-by: Alessandro Ratti <alessandro@0x65c.net>
> ---
>  arch/x86/kvm/x86.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index ff8812f3a129..d5cf9a7ff8c5 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -11596,7 +11596,15 @@ static inline int vcpu_block(struct kvm_vcpu *vcpu)
>         if (is_guest_mode(vcpu)) {
>                 int r = kvm_check_nested_events(vcpu);
>
> -               WARN_ON_ONCE(r == -EBUSY);
> +               /*
> +                * -EBUSY indicates a nested event is pending but cannot be
> +                * injected immediately (e.g., event delivery is temporarily
> +                * blocked). Return to the vCPU run loop to retry guest entry
> +                * instead of blocking, which would lose the pending event.
> +                */
> +               if (r == -EBUSY)
> +                       return 1;
> +
>                 if (r < 0)
>                         return 0;
>         }
> -- 
> 2.52.0

Re: [syzbot] [kvm?] WARNING in vcpu_run

[syzbot]

syzbot has bisected this issue to:

commit 45405155d876c326da89162b8173b8cc9ab7ed75
Author: Sean Christopherson <seanjc@google.com>
Date:   Fri Jun 7 17:26:09 2024 +0000

    KVM: x86: WARN if a vCPU gets a valid wakeup that KVM can't yet inject

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=11ac34df980000
start commit:   ae90f6a6170d Merge tag 'bpf-fixes' of git://git.kernel.org..
git tree:       upstream
final oops:     https://syzkaller.appspot.com/x/report.txt?x=13ac34df980000
console output: https://syzkaller.appspot.com/x/log.txt?x=15ac34df980000
kernel config:  https://syzkaller.appspot.com/x/.config?x=309bb816d40abc28
dashboard link: https://syzkaller.appspot.com/bug?extid=1522459a74d26b0ac33a
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=158d0230580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=12f8de40580000

Reported-by: syzbot+1522459a74d26b0ac33a@syzkaller.appspotmail.com
Fixes: 45405155d876 ("KVM: x86: WARN if a vCPU gets a valid wakeup that KVM can't yet inject")

For information about bisection process see: https://goo.gl/tpsmEJ#bisection

[syzbot] [kvm?] WARNING in vcpu_run

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    ae90f6a6170d Merge tag 'bpf-fixes' of git://git.kernel.org..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=168d0230580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=309bb816d40abc28
dashboard link: https://syzkaller.appspot.com/bug?extid=1522459a74d26b0ac33a
compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=158d0230580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=12f8de40580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/1fd044836856/disk-ae90f6a6.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/a01e01be8aa8/vmlinux-ae90f6a6.xz
kernel image: https://storage.googleapis.com/syzbot-assets/9b0e73e0cce7/bzImage-ae90f6a6.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+1522459a74d26b0ac33a@syzkaller.appspotmail.com

------------[ cut here ]------------
WARNING: CPU: 1 PID: 5838 at arch/x86/kvm/x86.c:11215 vcpu_block arch/x86/kvm/x86.c:11215 [inline]
WARNING: CPU: 1 PID: 5838 at arch/x86/kvm/x86.c:11215 vcpu_run+0x872d/0x8900 arch/x86/kvm/x86.c:11259
Modules linked in:
CPU: 1 UID: 0 PID: 5838 Comm: syz-executor929 Not tainted 6.12.0-rc4-syzkaller-00161-gae90f6a6170d #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024
RIP: 0010:vcpu_block arch/x86/kvm/x86.c:11215 [inline]
RIP: 0010:vcpu_run+0x872d/0x8900 arch/x86/kvm/x86.c:11259
Code: 48 3b 84 24 e0 04 00 00 0f 85 e5 01 00 00 44 89 f0 48 8d 65 d8 5b 41 5c 41 5d 41 5e 41 5f 5d c3 cc cc cc cc e8 f4 12 81 00 90 <0f> 0b 90 e9 72 ff ff ff e8 e6 12 81 00 e9 68 ff ff ff e8 dc 12 81
RSP: 0018:ffffc90003c6f480 EFLAGS: 00010293
RAX: ffffffff8113c4cc RBX: 00000000fffffff0 RCX: ffff88802eae8000
RDX: 0000000000000000 RSI: 00000000fffffff0 RDI: 00000000fffffff0
RBP: ffffc90003c6f9b0 R08: ffffffff8113498c R09: 1ffff110069638dc
R10: dffffc0000000000 R11: ffffed10069638dd R12: 1ffff1100691305d
R13: ffff888034898000 R14: ffffffff8e72ae90 R15: ffff888034898038
FS:  00007f148304f6c0(0000) GS:ffff8880b8700000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 00000000781b4000 CR4: 00000000003526f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 kvm_arch_vcpu_ioctl_run+0xa73/0x19d0 arch/x86/kvm/x86.c:11575
 kvm_vcpu_ioctl+0x91a/0xea0 virt/kvm/kvm_main.c:4475
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:907 [inline]
 __se_sys_ioctl+0xf9/0x170 fs/ioctl.c:893
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f14830f9049
Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 c1 17 00 00 90 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 b0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f148304f228 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f1483183358 RCX: 00007f14830f9049
RDX: 0000000000000000 RSI: 000000000000ae80 RDI: 0000000000000005
RBP: 00007f1483183350 R08: 00007ffe514601c7 R09: 00007f148304f6c0
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f148318335c
R13: 00007f1483150038 R14: 6d766b2f7665642f R15: 00007ffe514601c8
 </TASK>


---
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