[syzbot] [kvm?] WARNING in kvm_read_guest_offset_cached

[syzbot] [kvm?] WARNING in kvm_read_guest_offset_cached

[syzbot]

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_arch_can_dequeue_async_page_present+0x172/0x310 arch/x86/kvm/x86.c:13571
 kvm_check_async_pf_completion+0x61/0x440 virt/kvm/async_pf.c:158
 vcpu_enter_guest arch/x86/kvm/x86.c:10930 [inline]
 vcpu_run+0x48c2/0x5500 arch/x86/kvm/x86.c:11335
 kvm_arch_vcpu_ioctl_run+0x51e/0x18c0 arch/x86/kvm/x86.c:11658
 kvm_vcpu_ioctl+0x5eb/0x1690 virt/kvm/kvm_main.c:4464
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:907 [inline]
 __se_sys_ioctl fs/ioctl.c:893 [inline]
 __x64_sys_ioctl+0x18e/0x210 fs/ioctl.c:893
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xcd/0x4c0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fdb7658e929
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 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 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffd25fe7d98 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007fdb767b5fa0 RCX: 00007fdb7658e929
RDX: 0000000000000000 RSI: 000000000000ae80 RDI: 0000000000000005
RBP: 00007fdb76610b39 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fdb767b5fa0 R14: 00007fdb767b5fa0 R15: 0000000000000003
 </TASK>


---
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#syz undup

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

[Sean Christopherson]

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:

---
 arch/x86/kvm/x86.c | 32 ++++++++++++++++++--------------
 1 file changed, 18 insertions(+), 14 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a1c49bc681c4..0fbbf297b3c8 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3547,11 +3547,16 @@ static int set_msr_mce(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 	return 0;
 }
 
-static inline bool kvm_pv_async_pf_enabled(struct kvm_vcpu *vcpu)
+static bool __kvm_pv_async_pf_enabled(u64 msr_en_val)
 {
 	u64 mask = KVM_ASYNC_PF_ENABLED | KVM_ASYNC_PF_DELIVERY_AS_INT;
 
-	return (vcpu->arch.apf.msr_en_val & mask) == mask;
+	return (msr_en_val & mask) == mask;
+}
+
+static inline bool kvm_pv_async_pf_enabled(struct kvm_vcpu *vcpu)
+{
+	return __kvm_pv_async_pf_enabled(vcpu->arch.apf.msr_en_val);
 }
 
 static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data)
@@ -3573,22 +3578,21 @@ static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data)
 	if (!lapic_in_kernel(vcpu))
 		return data ? 1 : 0;
 
+	if (__kvm_pv_async_pf_enabled(data) &&
+	    kvm_gfn_to_hva_cache_init(vcpu->kvm, &vcpu->arch.apf.data, gpa,
+				      sizeof(u64)))
+		return 1;
+
 	vcpu->arch.apf.msr_en_val = data;
-
-	if (!kvm_pv_async_pf_enabled(vcpu)) {
-		kvm_clear_async_pf_completion_queue(vcpu);
-		kvm_async_pf_hash_reset(vcpu);
-		return 0;
-	}
-
-	if (kvm_gfn_to_hva_cache_init(vcpu->kvm, &vcpu->arch.apf.data, gpa,
-					sizeof(u64)))
-		return 1;
-
 	vcpu->arch.apf.send_always = (data & KVM_ASYNC_PF_SEND_ALWAYS);
 	vcpu->arch.apf.delivery_as_pf_vmexit = data & KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT;
 
-	kvm_async_pf_wakeup_all(vcpu);
+	if (kvm_pv_async_pf_enabled(vcpu)) {
+		kvm_clear_async_pf_completion_queue(vcpu);
+		kvm_async_pf_hash_reset(vcpu);
+	} else {
+		kvm_async_pf_wakeup_all(vcpu);
+	}
 
 	return 0;
 }

base-commit: 2a046f6a4ecce47ada50dba529ec726dd0d34351
--

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

[Alexander Potapenko]

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.

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

[Sean Christopherson]

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.

[PATCH] KVM: x86: Don't leave APF half-enabled on bad APF data GPA

[Ethan Yang]

kvm_pv_enable_async_pf() updates vcpu->arch.apf.msr_en_val before
initializing the APF data gfn_to_hva cache. If userspace provides an
invalid GPA, kvm_gfn_to_hva_cache_init() fails, but msr_en_val stays
enabled and leaves APF state half-initialized.

Later APF paths can then try to use the empty cache and trigger
WARN_ON() in kvm_read_guest_offset_cached().

Determine the new APF enabled state from the incoming MSR value, do cache
initialization first on the enable path, and commit msr_en_val only after
successful initialization. Keep the disable path behavior unchanged.

Reported-by: syzbot+bc0e18379a290e5edfe4@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=bc0e18379a290e5edfe4
Link: https://lore.kernel.org/r/aHfD3MczrDpzDX9O@google.com
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Ethan Yang <ethan.yang.kernel@gmail.com>
---
 arch/x86/kvm/x86.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index fd1c4a36b59..c355bbd36c3 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1041,11 +1041,15 @@ bool kvm_require_dr(struct kvm_vcpu *vcpu, int dr)
 }
 EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_require_dr);
 
-static bool kvm_pv_async_pf_enabled(struct kvm_vcpu *vcpu)
+static bool kvm_pv_async_pf_enabled_data(u64 data)
 {
 	u64 mask = KVM_ASYNC_PF_ENABLED | KVM_ASYNC_PF_DELIVERY_AS_INT;
+	return (data & mask) == mask;
+}
 
-	return (vcpu->arch.apf.msr_en_val & mask) == mask;
+static bool kvm_pv_async_pf_enabled(struct kvm_vcpu *vcpu)
+{
+	return kvm_pv_async_pf_enabled_data(vcpu->arch.apf.msr_en_val);
 }
 
 static inline u64 pdptr_rsvd_bits(struct kvm_vcpu *vcpu)
@@ -3645,17 +3649,20 @@ static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data)
 	if (!lapic_in_kernel(vcpu))
 		return data ? 1 : 0;
 
+	bool new_enabled = kvm_pv_async_pf_enabled_data(data);
+
+	if (new_enabled &&
+		kvm_gfn_to_hva_cache_init(vcpu->kvm, &vcpu->arch.apf.data, gpa,
+			sizeof(u64)))
+		return 1;
 	vcpu->arch.apf.msr_en_val = data;
 
-	if (!kvm_pv_async_pf_enabled(vcpu)) {
+	if (!new_enabled) {
 		kvm_clear_async_pf_completion_queue(vcpu);
 		kvm_async_pf_hash_reset(vcpu);
 		return 0;
 	}
 
-	if (kvm_gfn_to_hva_cache_init(vcpu->kvm, &vcpu->arch.apf.data, gpa,
-					sizeof(u64)))
-		return 1;
 
 	vcpu->arch.apf.send_always = (data & KVM_ASYNC_PF_SEND_ALWAYS);
 	vcpu->arch.apf.delivery_as_pf_vmexit = data & KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT;
-- 
2.47.3

Re: [PATCH] KVM: x86: Don't leave APF half-enabled on bad APF data GPA

[Xiaoyao Li]

On 4/3/2026 11:11 AM, Ethan Yang wrote:
> kvm_pv_enable_async_pf() updates vcpu->arch.apf.msr_en_val before
> initializing the APF data gfn_to_hva cache. If userspace provides an
> invalid GPA, kvm_gfn_to_hva_cache_init() fails, but msr_en_val stays
> enabled and leaves APF state half-initialized.
> 
> Later APF paths can then try to use the empty cache and trigger
> WARN_ON() in kvm_read_guest_offset_cached().
> 
> Determine the new APF enabled state from the incoming MSR value, do cache
> initialization first on the enable path, and commit msr_en_val only after
> successful initialization. Keep the disable path behavior unchanged.
> 
> Reported-by: syzbot+bc0e18379a290e5edfe4@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=bc0e18379a290e5edfe4
> Link: https://lore.kernel.org/r/aHfD3MczrDpzDX9O@google.com
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Ethan Yang <ethan.yang.kernel@gmail.com>

Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>

though some nits below.

> ---
>   arch/x86/kvm/x86.c | 19 +++++++++++++------
>   1 file changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index fd1c4a36b59..c355bbd36c3 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1041,11 +1041,15 @@ bool kvm_require_dr(struct kvm_vcpu *vcpu, int dr)
>   }
>   EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_require_dr);
>   
> -static bool kvm_pv_async_pf_enabled(struct kvm_vcpu *vcpu)
> +static bool kvm_pv_async_pf_enabled_data(u64 data)

I'm not sure if the name with "__" as __kvm_pv_async_pf_enabled() fits 
better?

>   {
>   	u64 mask = KVM_ASYNC_PF_ENABLED | KVM_ASYNC_PF_DELIVERY_AS_INT;
> +	return (data & mask) == mask;
> +}
>   
> -	return (vcpu->arch.apf.msr_en_val & mask) == mask;
> +static bool kvm_pv_async_pf_enabled(struct kvm_vcpu *vcpu)
> +{
> +	return kvm_pv_async_pf_enabled_data(vcpu->arch.apf.msr_en_val);
>   }
>   
>   static inline u64 pdptr_rsvd_bits(struct kvm_vcpu *vcpu)
> @@ -3645,17 +3649,20 @@ static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data)
>   	if (!lapic_in_kernel(vcpu))
>   		return data ? 1 : 0;
>   
> +	bool new_enabled = kvm_pv_async_pf_enabled_data(data);

I would like just call it "enable"

> +
> +	if (new_enabled &&
> +		kvm_gfn_to_hva_cache_init(vcpu->kvm, &vcpu->arch.apf.data, gpa,

Please align the second line

> +			sizeof(u64)))
> +		return 1;
>   	vcpu->arch.apf.msr_en_val = data;
>   
> -	if (!kvm_pv_async_pf_enabled(vcpu)) {
> +	if (!new_enabled) {
>   		kvm_clear_async_pf_completion_queue(vcpu);
>   		kvm_async_pf_hash_reset(vcpu);
>   		return 0;
>   	}
>   
> -	if (kvm_gfn_to_hva_cache_init(vcpu->kvm, &vcpu->arch.apf.data, gpa,
> -					sizeof(u64)))
> -		return 1;
>   
>   	vcpu->arch.apf.send_always = (data & KVM_ASYNC_PF_SEND_ALWAYS);
>   	vcpu->arch.apf.delivery_as_pf_vmexit = data & KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT;

[PATCH v2] KVM: x86: Don't leave APF half-enabled on bad APF data GPA

[Ethan Yang]

kvm_pv_enable_async_pf() updates vcpu->arch.apf.msr_en_val before
initializing the APF data gfn_to_hva cache. If userspace provides an
invalid GPA, kvm_gfn_to_hva_cache_init() fails, but msr_en_val stays
enabled and leaves APF state half-initialized.

Later APF paths can then try to use the empty cache and trigger
WARN_ON() in kvm_read_guest_offset_cached().

Determine the new APF enabled state from the incoming MSR value, do cache
initialization first on the enable path, and commit msr_en_val only after
successful initialization. Keep the disable path behavior unchanged.

Reported-by: syzbot+bc0e18379a290e5edfe4@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=bc0e18379a290e5edfe4
Link: https://lore.kernel.org/r/aHfD3MczrDpzDX9O@google.com
Suggested-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Ethan Yang <ethan.yang.kernel@gmail.com>
---
v2:
- Rename helper to __kvm_pv_async_pf_enabled()
- Rename new_enabled to enable
- Align wrapped line in kvm_gfn_to_hva_cache_init() call
---
 arch/x86/kvm/x86.c | 22 +++++++++++++++-------
 1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 63afdb6bb07..93b7ad9b52b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1035,11 +1035,16 @@ bool kvm_require_dr(struct kvm_vcpu *vcpu, int dr)
 }
 EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_require_dr);
 
-static bool kvm_pv_async_pf_enabled(struct kvm_vcpu *vcpu)
+static bool __kvm_pv_async_pf_enabled(u64 data)
 {
 	u64 mask = KVM_ASYNC_PF_ENABLED | KVM_ASYNC_PF_DELIVERY_AS_INT;
 
-	return (vcpu->arch.apf.msr_en_val & mask) == mask;
+	return (data & mask) == mask;
+}
+
+static bool kvm_pv_async_pf_enabled(struct kvm_vcpu *vcpu)
+{
+	return __kvm_pv_async_pf_enabled(vcpu->arch.apf.msr_en_val);
 }
 
 static inline u64 pdptr_rsvd_bits(struct kvm_vcpu *vcpu)
@@ -3616,6 +3621,7 @@ static int set_msr_mce(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data)
 {
 	gpa_t gpa = data & ~0x3f;
+	bool enable;
 
 	/* Bits 4:5 are reserved, Should be zero */
 	if (data & 0x30)
@@ -3632,18 +3638,20 @@ static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data)
 	if (!lapic_in_kernel(vcpu))
 		return data ? 1 : 0;
 
+	enable = __kvm_pv_async_pf_enabled(data);
+
+	if (enable &&
+	    kvm_gfn_to_hva_cache_init(vcpu->kvm, &vcpu->arch.apf.data, gpa,
+				      sizeof(u64)))
+		return 1;
 	vcpu->arch.apf.msr_en_val = data;
 
-	if (!kvm_pv_async_pf_enabled(vcpu)) {
+	if (!enable) {
 		kvm_clear_async_pf_completion_queue(vcpu);
 		kvm_async_pf_hash_reset(vcpu);
 		return 0;
 	}
 
-	if (kvm_gfn_to_hva_cache_init(vcpu->kvm, &vcpu->arch.apf.data, gpa,
-					sizeof(u64)))
-		return 1;
-
 	vcpu->arch.apf.send_always = (data & KVM_ASYNC_PF_SEND_ALWAYS);
 	vcpu->arch.apf.delivery_as_pf_vmexit = data & KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT;
 
-- 
2.47.3

Re: [PATCH v2] KVM: x86: Don't leave APF half-enabled on bad APF data GPA

[Sean Christopherson]

Thanks for posting this!  My "week" estimate was wee bit off...

On Fri, Apr 03, 2026, Ethan Yang wrote:
>  static inline u64 pdptr_rsvd_bits(struct kvm_vcpu *vcpu)
> @@ -3616,6 +3621,7 @@ static int set_msr_mce(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data)
>  {
>  	gpa_t gpa = data & ~0x3f;
> +	bool enable;
>  
>  	/* Bits 4:5 are reserved, Should be zero */
>  	if (data & 0x30)
> @@ -3632,18 +3638,20 @@ static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data)
>  	if (!lapic_in_kernel(vcpu))
>  		return data ? 1 : 0;
>  
> +	enable = __kvm_pv_async_pf_enabled(data);
> +
> +	if (enable &&
> +	    kvm_gfn_to_hva_cache_init(vcpu->kvm, &vcpu->arch.apf.data, gpa,
> +				      sizeof(u64)))

I would rather forgo a local variable and either hhave the below check stay as
kvm_pv_async_pf_enabled() or just redo the call to __kvm_pv_async_pf_enabled().

> +		return 1;

Newline please.

>  	vcpu->arch.apf.msr_en_val = data;
>  
> -	if (!kvm_pv_async_pf_enabled(vcpu)) {
> +	if (!enable) {
>  		kvm_clear_async_pf_completion_queue(vcpu);
>  		kvm_async_pf_hash_reset(vcpu);
>  		return 0;
>  	}
>  
> -	if (kvm_gfn_to_hva_cache_init(vcpu->kvm, &vcpu->arch.apf.data, gpa,
> -					sizeof(u64)))
> -		return 1;
> -
>  	vcpu->arch.apf.send_always = (data & KVM_ASYNC_PF_SEND_ALWAYS);
>  	vcpu->arch.apf.delivery_as_pf_vmexit = data & KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT;

As I sketched out, in a follow-up patch, I would like to to update these fields
as well.  I don't like tracking stale information, even if it _should_ be unused.

Actually, even better, just drop the fields.  That way zeroing msr_en_val via
INIT won't lead to stale data either.

I'll post a v3, should be easier overall than posting diffs for the suggestions
and then making you write changelogs :-)