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

Sean Christopherson
2022-05-12T13:48:34Z

On Thu, May 12, 2022, Vitaly Kuznetsov wrote:
> Maxim Levitsky <mlevitsk@redhat.com> writes:
>
> > Fixes: 1c2361f667f36 ("KVM: x86: Use __try_cmpxchg_user() to emulate atomic accesses")
> > Signed-off-by: Maxim Levitsky <mlevitsk@redhat.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@redhat.com>

Yep, if this doesn't get squashed

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