2022-05-13 19:41:39

by Sean Christopherson

Subject: [PATCH 1/3] KVM: x86: Signal #GP, not -EPERM, on bad WRMSR(MCi_CTL/STATUS)

Return '1', not '-1', when handling an illegal WRMSR to a MCi_CTL or
MCi_STATUS MSR. The behavior of "all zeros' or "all ones" for CTL MSRs
is architectural, as is the "only zeros" behavior for STATUS MSRs. I.e.
the intent is to inject a #GP, not exit to userspace due to an unhandled
emulation case. Returning '-1' gets interpreted as -EPERM up the stack
and effecitvely kills the guest.

Fixes: 890ca9aefa78 ("KVM: Add MCE support")
Fixes: 9ffd986c6e4e ("KVM: X86: #GP when guest attempts to write MCi_STATUS register w/o 0")
Cc: [email protected]
Signed-off-by: Sean Christopherson <[email protected]>
---
arch/x86/kvm/x86.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 8ee8c91fa762..bc6db58975dc 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3233,13 +3233,13 @@ static int set_msr_mce(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
*/
if ((offset & 0x3) == 0 &&
data != 0 && (data | (1 << 10)) != ~(u64)0)
- return -1;
+ return 1;

/* MCi_STATUS */
if (!msr_info->host_initiated &&
(offset & 0x3) == 1 && data != 0) {
if (!can_set_mci_status(vcpu))
- return -1;
+ return 1;
}

vcpu->arch.mce_banks[offset] = data;
--
2.36.0.550.gb090851708-goog

2022-05-14 01:46:37

by Jim Mattson

Subject: Re: [PATCH 1/3] KVM: x86: Signal #GP, not -EPERM, on bad WRMSR(MCi_CTL/STATUS)

On Thu, May 12, 2022 at 3:27 PM Sean Christopherson <[email protected]> wrote:
>
> Return '1', not '-1', when handling an illegal WRMSR to a MCi_CTL or
> MCi_STATUS MSR. The behavior of "all zeros' or "all ones" for CTL MSRs
> is architectural, as is the "only zeros" behavior for STATUS MSRs. I.e.
> the intent is to inject a #GP, not exit to userspace due to an unhandled
> emulation case. Returning '-1' gets interpreted as -EPERM up the stack
> and effecitvely kills the guest.
>
> Fixes: 890ca9aefa78 ("KVM: Add MCE support")
> Fixes: 9ffd986c6e4e ("KVM: X86: #GP when guest attempts to write MCi_STATUS register w/o 0")
> Cc: [email protected]
> Signed-off-by: Sean Christopherson <[email protected]>
> ---
Reviewed-by: Jim Mattson <[email protected]>