Re: [PATCH] KVM: x86: revalidate steal time cache if MSR value changes

[Dr. David Alan Gilbert]

* Paolo Bonzini (pbonzini@xxxxxxxxxx) wrote:
> 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 steal
> time data is written at the address used by the old kernel instead of
> the old one.
> 
> While at it, rename the variable from gfn to gpa since it is a plain
> physical address and not a right-shifted one.
> 
> Reported-by: Dave Young <ruyang@xxxxxxxxxx>
> Reported-by: Xiaoying Yan  <yiyan@xxxxxxxxxx>
> Analyzed-by: Dr. David Alan Gilbert <dgilbert@xxxxxxxxxx>

The kernel you built with this in passes Dave Young's kexec set I was
using to debug it, so:

Tested-by: Dr. David Alan Gilbert <dgilbert@xxxxxxxxxx>

> 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>
> ---
>  arch/x86/kvm/x86.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e5fa335a4ea7..36dcf18b04bf 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3380,6 +3380,7 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
>  	struct gfn_to_hva_cache *ghc = &vcpu->arch.st.cache;
>  	struct kvm_steal_time __user *st;
>  	struct kvm_memslots *slots;
> +	gpa_t gpa = vcpu->arch.st.msr_val & KVM_STEAL_VALID_BITS;
>  	u64 steal;
>  	u32 version;
>  
> @@ -3397,13 +3398,12 @@ static void record_steal_time(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)) {
> -		gfn_t gfn = vcpu->arch.st.msr_val & KVM_STEAL_VALID_BITS;
> -
>  		/* We rely on the fact that it fits in a single page. */
>  		BUILD_BUG_ON((sizeof(*st) - 1) & KVM_STEAL_VALID_BITS);
>  
> -		if (kvm_gfn_to_hva_cache_init(vcpu->kvm, ghc, gfn, sizeof(*st)) ||
> +		if (kvm_gfn_to_hva_cache_init(vcpu->kvm, ghc, gpa, sizeof(*st)) ||
>  		    kvm_is_error_hva(ghc->hva) || !ghc->memslot)
>  			return;
>  	}
> -- 
> 2.37.1
> 
-- 
Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK