Re: [PATCH] KVM: x86: do not report preemption if the steal time cache is stale

[Vitaly Kuznetsov]

Paolo Bonzini <pbonzini@xxxxxxxxxx> writes:

> Commit 7e2175ebd695 ("KVM: x86: Fix recording of guest steal time
> / preempted status", 2021-11-11) open coded the previous call to
> kvm_map_gfn, but in doing so it dropped the comparison between the cached
> guest physical address and the one in the MSR.  This cause an incorrect
> cache hit if the guest modifies the steal time address while the memslots
> remain the same.  This can happen with kexec, in which case the preempted
> bit is written at the address used by the old kernel instead of
> the old one.
>
> Cc: David Woodhouse <dwmw@xxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 7e2175ebd695 ("KVM: x86: Fix recording of guest steal time / preempted status")
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

No need to S-o-b twice)

> ---
>  arch/x86/kvm/x86.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 0f3c2e034740..8ee4698cb90a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4715,6 +4715,7 @@ static void kvm_steal_time_set_preempted(struct kvm_vcpu *vcpu)
>  	struct kvm_steal_time __user *st;
>  	struct kvm_memslots *slots;
>  	static const u8 preempted = KVM_VCPU_PREEMPTED;
> +	gpa_t gpa = vcpu->arch.st.msr_val & KVM_STEAL_VALID_BITS;
>  
>  	/*
>  	 * The vCPU can be marked preempted if and only if the VM-Exit was on
> @@ -4742,6 +4743,7 @@ static void kvm_steal_time_set_preempted(struct kvm_vcpu *vcpu)
>  	slots = kvm_memslots(vcpu->kvm);
>  
>  	if (unlikely(slots->generation != ghc->generation ||
> +		     gpa != ghc->gpa ||
>  		     kvm_is_error_hva(ghc->hva) || !ghc->memslot))

(We could probably have a common helper for both these places.)

>  		return;

Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>

-- 
Vitaly