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Date:   Wed, 16 Oct 2019 11:47:03 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Paolo Bonzini <pbonzini@redhat.com>
cc:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Fenghua Yu <fenghua.yu@intel.com>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        H Peter Anvin <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Radim Krcmar <rkrcmar@redhat.com>,
        Ashok Raj <ashok.raj@intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Xiaoyao Li <xiaoyao.li@intel.com>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>,
        Ravi V Shankar <ravi.v.shankar@intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        x86 <x86@kernel.org>, kvm@vger.kernel.org
Subject: Re: [PATCH v9 09/17] x86/split_lock: Handle #AC exception for split
 lock
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On Wed, 16 Oct 2019, Paolo Bonzini wrote:
> On 25/09/19 20:09, Sean Christopherson wrote:
> >   - Remove KVM loading of MSR_TEST_CTRL, i.e. KVM *never* writes the CPU's
> >     actual MSR_TEST_CTRL.  KVM still emulates MSR_TEST_CTRL so that the
> >     guest can do WRMSR and handle its own #AC faults, but KVM doesn't
> >     change the value in hardware.
> > 
> >       * Allowing guest to enable split-lock detection can induce #AC on
> >         the host after it has been explicitly turned off, e.g. the sibling
> >         hyperthread hits an #AC in the host kernel, or worse, causes a
> >         different process in the host to SIGBUS.
> > 
> >       * Allowing guest to disable split-lock detection opens up the host
> >         to DoS attacks.
> > 
> >   - KVM advertises split-lock detection to guest/userspace if and only if
> >     split_lock_detect_disabled is zero.
> > 
> >   - Add a pr_warn_once() in KVM that triggers if split locks are disabled
> >     after support has been advertised to a guest.
> > 
> > Does this sound sane?
> 
> Not really, unfortunately.  Just never advertise split-lock detection to
> guests.  If the host has enabled split-lock detection, trap #AC and
> forward it to the host handler---which would disable split lock
> detection globally and reenter the guest.

Which completely defeats the purpose.

1) Sane guest

   Guest kernel has #AC handler and you basically prevent it from detecting
   malicious user space and killing it. You also prevent #AC detection in
   the guest kernel which limits debugability.

2) Malicious guest

   Trigger #AC to disable the host detection and then carry out the DoS
   attack.

Try again.

Thanks,

	tglx