KVM: arm64: Prevent access to vCPU events before init

PATCH Applied Oliver Upton 2 participants 3 messages Sep 30, 2025

Subsystems (3)

Stable branch arm64 KVM arm64

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Oliver Upton

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2025-09-30 08:52 UTC #0 lore raw

PATCH 1/1 KVM: arm64: Prevent access to vCPU events before init

Another day, another syzkaller bug. KVM erroneously allows userspace to pend vCPU events for a vCPU that hasn't been initialized yet, leading to KVM interpreting a bunch of uninitialized garbage for routing / injecting the exception.

In one case the injection code and the hyp disagree on whether the vCPU has a 32bit EL1 and put the vCPU into an illegal mode for AArch64, tripping the BUG() in exception_target_el() during the next injection:

kernel BUG at arch/arm64/kvm/inject_fault.c:40!

  Internal error: Oops - BUG: 00000000f2000800 [#1]  SMP
  CPU: 3 UID: 0 PID: 318 Comm: repro Not tainted 6.17.0-rc4-00104-g10fd0285305d #6 PREEMPT
  Hardware name: linux,dummy-virt (DT)
  pstate: 21402009 (nzCv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=--)
  pc : exception_target_el+0x88/0x8c
  lr : pend_serror_exception+0x18/0x13c
  sp : ffff800082f03a10
  x29: ffff800082f03a10 x28: ffff0000cb132280 x27: 0000000000000000
  x26: 0000000000000000 x25: ffff0000c2a99c20 x24: 0000000000000000
  x23: 0000000000008000 x22: 0000000000000002 x21: 0000000000000004
  x20: 0000000000008000 x19: ffff0000c2a99c20 x18: 0000000000000000
  x17: 0000000000000000 x16: 0000000000000000 x15: 00000000200000c0
  x14: 0000000000000000 x13: 0000000000000000 x12: 0000000000000000
  x11: 0000000000000000 x10: 0000000000000000 x9 : 0000000000000000
  x8 : ffff800082f03af8 x7 : 0000000000000000 x6 : 0000000000000000
  x5 : ffff800080f621f0 x4 : 0000000000000000 x3 : 0000000000000000
  x2 : 000000000040009b x1 : 0000000000000003 x0 : ffff0000c2a99c20
  Call trace:
   exception_target_el+0x88/0x8c (P)
   kvm_inject_serror_esr+0x40/0x3b4
   __kvm_arm_vcpu_set_events+0xf0/0x100
   kvm_arch_vcpu_ioctl+0x180/0x9d4
   kvm_vcpu_ioctl+0x60c/0x9f4
   __arm64_sys_ioctl+0xac/0x104
   invoke_syscall+0x48/0x110
   el0_svc_common.constprop.0+0x40/0xe0
   do_el0_svc+0x1c/0x28
   el0_svc+0x34/0xf0
   el0t_64_sync_handler+0xa0/0xe4
   el0t_64_sync+0x198/0x19c
  Code: f946bc01 b4fffe61 9101e020 17fffff2 (d4210000)

Reject the ioctls outright as no sane VMM would call these before KVM_ARM_VCPU_INIT anyway. Even if it did the exception would've been thrown away by the eventual reset of the vCPU's state.

Cc [email protected] # 6.17

Fixes b7b27facc7b5 ("arm/arm64: KVM: Add KVM_GET/SET_VCPU_EVENTS")

Signed-off-by Oliver Upton <[email protected]>

---

While the blamed commit is indeed broken, only 6.17+ kernels actually hit the BUG() due to commit efa1368ba9f4 ("KVM: arm64: Commit exceptions from KVM_SET_VCPU_EVENTS immediately).

 arch/arm64/kvm/arm.c | 6 ++++++
 1 file changed, 6 insertions(+)

arch/arm64/kvm/arm.c +6 -0

index a59b4046617c..c44357d26ee8 100644
@@ -1795,6 +1795,9 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
 	case KVM_GET_VCPU_EVENTS: {
 		struct kvm_vcpu_events events;
 
+		if (!kvm_vcpu_initialized(vcpu))
+			return -ENOEXEC;
+
 		if (kvm_arm_vcpu_get_events(vcpu, &events))
 			return -EINVAL;

@@ -1806,6 +1809,9 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
 	case KVM_SET_VCPU_EVENTS: {
 		struct kvm_vcpu_events events;
 
+		if (!kvm_vcpu_initialized(vcpu))
+			return -ENOEXEC;
+
 		if (copy_from_user(&events, argp, sizeof(events)))
 			return -EFAULT;

base-commit: 10fd0285305d0b48e8a3bf15d4f17fc4f3d68cb6

Signature · 1 line

2.39.5

Marc Zyngier

maintainer

2025-09-30 09:26 UTC #1 lore raw

Quoted 75 lines · Oliver Upton

Another day, another syzkaller bug. KVM erroneously allows userspace to pend vCPU events for a vCPU that hasn't been initialized yet, leading to KVM interpreting a bunch of uninitialized garbage for routing / injecting the exception.
In one case the injection code and the hyp disagree on whether the vCPU has a 32bit EL1 and put the vCPU into an illegal mode for AArch64, tripping the BUG() in exception_target_el() during the next injection:
kernel BUG at arch/arm64/kvm/inject_fault.c:40!
  Internal error: Oops - BUG: 00000000f2000800 [#1]  SMP
  CPU: 3 UID: 0 PID: 318 Comm: repro Not tainted 6.17.0-rc4-00104-g10fd0285305d #6 PREEMPT
  Hardware name: linux,dummy-virt (DT)
  pstate: 21402009 (nzCv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=--)
  pc : exception_target_el+0x88/0x8c
  lr : pend_serror_exception+0x18/0x13c
  sp : ffff800082f03a10
  x29: ffff800082f03a10 x28: ffff0000cb132280 x27: 0000000000000000
  x26: 0000000000000000 x25: ffff0000c2a99c20 x24: 0000000000000000
  x23: 0000000000008000 x22: 0000000000000002 x21: 0000000000000004
  x20: 0000000000008000 x19: ffff0000c2a99c20 x18: 0000000000000000
  x17: 0000000000000000 x16: 0000000000000000 x15: 00000000200000c0
  x14: 0000000000000000 x13: 0000000000000000 x12: 0000000000000000
  x11: 0000000000000000 x10: 0000000000000000 x9 : 0000000000000000
  x8 : ffff800082f03af8 x7 : 0000000000000000 x6 : 0000000000000000
  x5 : ffff800080f621f0 x4 : 0000000000000000 x3 : 0000000000000000
  x2 : 000000000040009b x1 : 0000000000000003 x0 : ffff0000c2a99c20
  Call trace:
   exception_target_el+0x88/0x8c (P)
   kvm_inject_serror_esr+0x40/0x3b4
   __kvm_arm_vcpu_set_events+0xf0/0x100
   kvm_arch_vcpu_ioctl+0x180/0x9d4
   kvm_vcpu_ioctl+0x60c/0x9f4
   __arm64_sys_ioctl+0xac/0x104
   invoke_syscall+0x48/0x110
   el0_svc_common.constprop.0+0x40/0xe0
   do_el0_svc+0x1c/0x28
   el0_svc+0x34/0xf0
   el0t_64_sync_handler+0xa0/0xe4
   el0t_64_sync+0x198/0x19c
  Code: f946bc01 b4fffe61 9101e020 17fffff2 (d4210000)
Reject the ioctls outright as no sane VMM would call these before KVM_ARM_VCPU_INIT anyway. Even if it did the exception would've been thrown away by the eventual reset of the vCPU's state.
Cc [email protected] # 6.17
Fixes b7b27facc7b5 ("arm/arm64: KVM: Add KVM_GET/SET_VCPU_EVENTS")
Signed-off-by Oliver Upton <[email protected]>
---
While the blamed commit is indeed broken, only 6.17+ kernels actually hit the BUG() due to commit efa1368ba9f4 ("KVM: arm64: Commit exceptions from KVM_SET_VCPU_EVENTS immediately).
 arch/arm64/kvm/arm.c | 6 ++++++
 1 file changed, 6 insertions(+)
arch/arm64/kvm/arm.c +3 -0
index a59b4046617c..c44357d26ee8 100644
@@ -1795,6 +1795,9 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
 	case KVM_GET_VCPU_EVENTS: {
 		struct kvm_vcpu_events events;
 
+		if (!kvm_vcpu_initialized(vcpu))
+			return -ENOEXEC;
+
 		if (kvm_arm_vcpu_get_events(vcpu, &events))
 			return -EINVAL;
 

arch/arm64/kvm/arm.c long kvm_arch_vcpu_ioctl(struct file *filp, · line 1817

Quoted 9 lines

arch/arm64/kvm/arm.c +3 -0

@@ -1806,6 +1809,9 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
 	case KVM_SET_VCPU_EVENTS: {
 		struct kvm_vcpu_events events;
 
+		if (!kvm_vcpu_initialized(vcpu))
+			return -ENOEXEC;
+
 		if (copy_from_user(&events, argp, sizeof(events)))
 			return -EFAULT;

LGTM, but the -ENOEXEC part will eventually need being documented. I'll queue this now though, as it fixes an annoying bug.

Thanks,

M.

Signature · 1 line

Without deviation from the norm, progress is not possible.

Marc Zyngier

maintainer Applied

2025-09-30 09:28 UTC #2 lore raw

In reply to Oliver Upton

Quoted 10 lines · Oliver Upton

Another day, another syzkaller bug. KVM erroneously allows userspace to pend vCPU events for a vCPU that hasn't been initialized yet, leading to KVM interpreting a bunch of uninitialized garbage for routing / injecting the exception.
In one case the injection code and the hyp disagree on whether the vCPU has a 32bit EL1 and put the vCPU into an illegal mode for AArch64, tripping the BUG() in exception_target_el() during the next injection:
[...]

Applied to fixes, thanks!

[1/1] KVM: arm64: Prevent access to vCPU events before init

      commit: cc96679f3c0348bf8450a5c84b71bb1351c027f9

Cheers,

M.

Signature · 1 line

Without deviation from the norm, progress is not possible.