linux-kernel - Forwarded: Private message regarding: [syzbot] [kvmarm?] WARNING in pend_serror

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Message-ID: <6872ff6f.a70a0220.3b380f.0014.GAE@google.com>
Date: Sat, 12 Jul 2025 17:35:59 -0700
From: syzbot <syzbot+1f6f096afda6f4f8f565@...kaller.appspotmail.com>
To: linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Forwarded: Private message regarding: [syzbot] [kvmarm?] WARNING in pend_serror_exception

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linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com.

***

Subject: Private message regarding: [syzbot] [kvmarm?] WARNING in pend_serror_exception
Author: kapoorarnav43@...il.com

#syz test

From: Arnav Kapoor <kapoorarnnav43@...il.com>
Date: Sat, 13 Jul 2025 20:30:00 +0000
Subject: [PATCH] arm64: KVM: Fix SError ESR validation to only allow ISS 
field

The current validation in __kvm_arm_vcpu_set_events incorrectly allows
userspace to set the ISV bit (bit 24) in the SError ESR when injecting
a SError through KVM_SET_VCPU_EVENTS. According to the ARM ARM and the
KVM API documentation, userspace should only be able to control the ISS
field (bits 23:0) of the ESR, not the ISV bit which is managed by hardware.

The problematic check:
  if (!((events->exception.serror_esr) & ~ESR_ELx_ISS_MASK))
      kvm_set_sei_esr(vcpu, events->exception.serror_esr);
  else
      return -EINVAL;

allows the ISV bit because ESR_ELx_ISS_MASK includes bit 24 (GENMASK(24,0)).
However, the ISV bit should be set only by KVM itself when calling 
kvm_set_sei_esr(), and userspace should not be able to control it.

Fix this by changing the validation to only allow bits 23:0 (the actual
ISS field without the ISV bit):

  if (events->exception.serror_esr & ~GENMASK(23, 0))
      return -EINVAL;

This prevents userspace from setting reserved bits and the ISV bit while
still allowing control over the intended ISS field. The condition is
simplified since we now have a straightforward check for invalid bits.

Reported-by: syzbot+1f6f096afda6f4f8f565@...kaller.appspotmail.com
Fixes: 0e5b9065dcf3 ("KVM: arm64: Inject SError exception into guest")
Signed-off-by: Arnav Kapoor <kapoorarnnav43@...il.com>
---
 arch/arm64/kvm/guest.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
index 2196979a24a3..cbe1e310f477 100644
--- a/arch/arm64/kvm/guest.c
+++ b/arch/arm64/kvm/guest.c
@@ -844,10 +844,10 @@ int __kvm_arm_vcpu_set_events(struct kvm_vcpu *vcpu,
  if (!cpus_have_final_cap(ARM64_HAS_RAS_EXTN))
  return -EINVAL;

- if (!((events->exception.serror_esr) & ~ESR_ELx_ISS_MASK))
- kvm_set_sei_esr(vcpu, events->exception.serror_esr);
- else
+ if (events->exception.serror_esr & ~GENMASK(23, 0))
  return -EINVAL;
+
+ kvm_set_sei_esr(vcpu, events->exception.serror_esr);
  } else if (serror_pending) {
  kvm_inject_vabt(vcpu);
  }
-- 
2.43.0

On Sunday, 13 July 2025 at 05:58:05 UTC+5:30 syzbot wrote:

Hello, 

syzbot tried to test the proposed patch but the build/boot failed: 

failed to apply patch: 
checking file arch/arm64/kvm/guest.c 
Hunk #1 FAILED at 844. 
1 out of 1 hunk FAILED 

Tested on: 

commit: 15724a98 Merge branch 'kvm-arm64/doublefault2' into kv.. 
git tree: git://git.kernel.org/pub/scm/linux/kernel/git/kvmarm/kvmarm.git 
next 
kernel config: https://syzkaller.appspot.com/x/.config?x=82bd3cd421993314 
dashboard link: https://syzkaller.appspot.com/bug?extid=1f6f096afda6f4f8f565 
compiler: 
userspace arch: arm64 
patch: https://syzkaller.appspot.com/x/patch.diff?x=10644e8c580000