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

[Sean Christopherson <seanjc@google.com>]

On Tue, Feb 10, 2026, Alexander Potapenko wrote:
> On Wed, Jul 16, 2025 at 5:23 PM 'Sean Christopherson' via
> syzkaller-bugs <syzkaller-bugs@googlegroups.com> wrote:
> >
> > On Tue, Jul 15, 2025, syzbot wrote:
> > > Hello,
> > >
> > > syzbot found the following issue on:
> > >
> > > HEAD commit:    155a3c003e55 Merge tag 'for-6.16/dm-fixes-2' of git://git...
> > > git tree:       upstream
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=103e858c580000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=8d5ef2da1e1c848
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=bc0e18379a290e5edfe4
> > > compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=153188f0580000
> > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16f6198c580000
> > >
> > > Downloadable assets:
> > > disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-155a3c00.raw.xz
> > > vmlinux: https://storage.googleapis.com/syzbot-assets/725a320dfe66/vmlinux-155a3c00.xz
> > > kernel image: https://storage.googleapis.com/syzbot-assets/9f06899bb6f3/bzImage-155a3c00.xz
> > >
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+bc0e18379a290e5edfe4@syzkaller.appspotmail.com
> > >
> > > ------------[ cut here ]------------
> > > WARNING: CPU: 0 PID: 6107 at arch/x86/kvm/../../../virt/kvm/kvm_main.c:3459 kvm_read_guest_offset_cached+0x3f5/0x4b0 virt/kvm/kvm_main.c:3459
> > > Modules linked in:
> > > CPU: 0 UID: 0 PID: 6107 Comm: syz.0.16 Not tainted 6.16.0-rc6-syzkaller-00002-g155a3c003e55 #0 PREEMPT(full)
> > > Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
> > > RIP: 0010:kvm_read_guest_offset_cached+0x3f5/0x4b0 virt/kvm/kvm_main.c:3459
> > > Code: 0f 01 e8 3e 6c 61 00 e9 9b fc ff ff e8 14 25 85 00 48 8b 3c 24 31 d2 48 89 ee e8 16 bf fa 00 e9 2e fe ff ff e8 fc 24 85 00 90 <0f> 0b 90 bb ea ff ff ff e9 4d fe ff ff e8 e9 24 85 00 48 8b 74 24
> > > RSP: 0018:ffffc9000349f960 EFLAGS: 00010293
> > > RAX: 0000000000000000 RBX: ffff888050329898 RCX: ffffffff8136ca66
> > > RDX: ffff88803cfa8000 RSI: ffffffff8136cd84 RDI: 0000000000000006
> > > RBP: 0000000000000004 R08: 0000000000000006 R09: 0000000000000008
> > > R10: 0000000000000000 R11: 0000000000000001 R12: 0000000000000004
> > > R13: ffffc90003921000 R14: 0000000000000000 R15: ffffc900039215a0
> > > FS:  000055558378f500(0000) GS:ffff8880d6713000(0000) knlGS:0000000000000000
> > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > CR2: 0000000000000000 CR3: 0000000025de6000 CR4: 0000000000352ef0
> > > Call Trace:
> > >  <TASK>
> > >  apf_pageready_slot_free arch/x86/kvm/x86.c:13452 [inline]
> >
> > kvm_pv_enable_async_pf() sets vcpu->arch.apf.msr_en_val even if the gpa is bad,
> > which leaves the cache in an empty state.  Something like so over a few patches
> > fixes the problem:
> 
> This bug is still occasionally reproducible on an Intel host running 6.19.

Gah, I never actually posted a series to fix this.  I'll try to get that done
this week.