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Message-ID: <aHfD3MczrDpzDX9O@google.com>
Date: Wed, 16 Jul 2025 08:23:08 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: syzbot <syzbot+bc0e18379a290e5edfe4@...kaller.appspotmail.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, pbonzini@...hat.com,
syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [kvm?] WARNING in kvm_read_guest_offset_cached
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@...kaller.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
--
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