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: <YoJZsJvq3YQ4xTWN@google.com>
Date:   Mon, 16 May 2022 14:03:28 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...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>,
        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 Mon, May 16, 2022, Maxim Levitsky wrote:
> 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.

Yeah, it should hit stable trees because of the explicit stable@.  The Fixes: on
this patch is likely enough, but no harm in being paranoid.

> > 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.

It mostly shows up in KUT, one of the tests deliberately triggers the scenario.
But yeah, there's definitely potential for breakage.  Not sure if a capability or
debug oriented module param would be best.  In theory, userspace could do a better
job of emulating the atomic access than KVM, which makes me lean toward a capability,
but practically speaking I doubt a userspace will ever do anything besides
terminate the guest.