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

#syz test

rom: Arnav Kapoor <kapoorarnnav43@gmail.com>
Date: Sat, 13 Jul 2025 21:30:00 +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 uses ESR_ELx_ISS_MASK which includes bit 24, allowing
userspace to control the ISV bit when it should only control bits 23:0.

This creates a potential security issue where userspace can manipulate the
ISV bit to potentially confuse or exploit kernel handling of SError 
exceptions.

Fix this by changing the validation to use GENMASK(23, 0) which explicitly
masks only the ISS field, preventing userspace from setting the ISV bit or
any other reserved bits beyond the ISS field.

The fix changes the logic from:
  if (!((events->exception.serror_esr) & ~ESR_ELx_ISS_MASK))
to:
  if (events->exception.serror_esr & ~GENMASK(23, 0))
      return -EINVAL;

This ensures that any attempt to set bits beyond the ISS field (including
the ISV bit) will be rejected with -EINVAL.

Reported-by: syzbot+1f6f096afda6f4f8f565@syzkaller.appspotmail.com
Fixes: 0e5b9065dcf3 ("KVM: arm64: Inject SError exception into guest")
Cc: stable@vger.kernel.org
Signed-off-by: Arnav Kapoor <kapoorarnnav43@gmail.com>
---
 arch/arm64/kvm/guest.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
index 12345678..87654321 100644
--- a/arch/arm64/kvm/guest.c
+++ b/arch/arm64/kvm/guest.c
@@ -844,10 +844,9 @@ int __kvm_arm_vcpu_set_events(struct kvm_vcpu *vcpu,
  if (serror_pending && has_esr) {
  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);
  }
-- 
2.40.1


On Sunday, 13 July 2025 at 06:13:04 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=153d90f0580000