linux-kernel - Re: [PATCH] KVM: x86: fix a typo in __try_cmpxchg_user that caused cmpxchg to be not atomic

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Message-ID: <Yn0QKHKKibBP2V0F@google.com>
Date:   Thu, 12 May 2022 13:48:24 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org,
        Wanpeng Li <wanpengli@...cent.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>, linux-kernel@...r.kernel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>, x86@...nel.org,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] KVM: x86: fix a typo in __try_cmpxchg_user that caused
 cmpxchg to be not atomic

On Thu, May 12, 2022, Vitaly Kuznetsov wrote:
> Maxim Levitsky <mlevitsk@...hat.com> writes:
> 
> > Fixes: 1c2361f667f36 ("KVM: x86: Use __try_cmpxchg_user() to emulate atomic accesses")
> > Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
> > ---
> >  arch/x86/kvm/x86.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > Yes, this is the root cause of the TDP mmu leak I was doing debug of in the last week.
> > Non working cmpxchg on which TDP mmu relies makes it install two differnt shadow pages
> > under same spte.

Ewww, as in running a buggy L0 resulted in a CMPXCHG going sideways in L1?  That's
awful.  My apologies :-(

> In case the fix is not squashed with 1c2361f667f36, the above should
> really go before '---'.
> 
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 8ee8c91fa7625..79cabd3d97d22 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -7329,7 +7329,7 @@ static int emulator_cmpxchg_emulated(struct x86_emulate_ctxt *ctxt,
> >  		goto emul_write;
> >  
> >  	hva = kvm_vcpu_gfn_to_hva(vcpu, gpa_to_gfn(gpa));
> > -	if (kvm_is_error_hva(addr))
> > +	if (kvm_is_error_hva(hva))
> 
> Looks like a typo indeed, so
> 
> Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

Yep, if this doesn't get squashed

Reviewed-by: Sean Christopherson <seanjc@...gle.com>