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: <f05dcf66ed2bfb7d113ce0d9a261569959265c68.camel@redhat.com>
Date:   Mon, 16 May 2022 16:14:31 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     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>,
        Jim Mattson <jmattson@...gle.com>, x86@...nel.org,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        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, 2022-05-12 at 21:27 +0000, Sean Christopherson wrote:
> On Thu, May 12, 2022, Paolo Bonzini wrote:
> > On 5/12/22 12:14, Maxim Levitsky wrote:
> > > 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.
> > 
> > Awesome!  And queued, thanks.
> 
> If you haven't done so already, can you add 
> 
>   Cc: stable@...r.kernel.org

When I posted my patch, I checked that the patch didn't reach mainline yet,
so I assumed that it won't be in -stable either yet, although it was CCed there.


> 
> Also, given that we have concrete proof that not honoring atomic accesses can have
> dire consequences for the guest, what about adding a capability to turn the emul_write
> path into an emulation error?
> 


This is a good idea. It might though break some guests - I did see that
warning few times, that is why I wasn't alert by the fact that it started showing up more often.

I'll take a look at this soon.

Best regards,
	Maxim Levitsky