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

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Message-ID: <Yuv9BoFtf9q3Ew5G@work-vm>
Date:   Thu, 4 Aug 2022 18:08:22 +0100
From:   "Dr. David Alan Gilbert" <dgilbert@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Dave Young <ruyang@...hat.com>,
        Xiaoying Yan <yiyan@...hat.com>,
        David Woodhouse <dwmw@...zon.co.uk>, stable@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: revalidate steal time cache if MSR value
 changes

* Vitaly Kuznetsov (vkuznets@...hat.com) wrote:
> Paolo Bonzini <pbonzini@...hat.com> 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 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@...hat.com>
> > Reported-by: Xiaoying Yan  <yiyan@...hat.com>
> > Analyzed-by: Dr. David Alan Gilbert <dgilbert@...hat.com>
> > Cc: David Woodhouse <dwmw@...zon.co.uk>
> > Cc: stable@...r.kernel.org
> > Fixes: 7e2175ebd695 ("KVM: x86: Fix recording of guest steal time / preempted status")
> > Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> > ---
> >  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)) ||
> 
> (It would be nice to somehow get at least a warning when 'gfn_t' is used
> instead of 'gpa_t' and vice versa)

Can't sparse be taught to do that?

Dave

> >  		    kvm_is_error_hva(ghc->hva) || !ghc->memslot)
> >  			return;
> >  	}
> 
> Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> 
> -- 
> Vitaly
> 
-- 
Dr. David Alan Gilbert / dgilbert@...hat.com / Manchester, UK