linux-kernel - Re: [PATCH v2 6/8] KVM: x86: Bug the VM if the emulator accesses a non-existent GPR

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Date:   Tue, 31 May 2022 11:06:07 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Robert Dinse <nanook@...imo.com>
Subject: Re: [PATCH v2 6/8] KVM: x86: Bug the VM if the emulator accesses a
 non-existent GPR

On Thu, May 26, 2022 at 09:08:15PM +0000, Sean Christopherson wrote:
> Bug the VM, i.e. kill it, if the emulator accesses a non-existent GPR,
> i.e. generates an out-of-bounds GPR index.  Continuing on all but
> gaurantees some form of data corruption in the guest, e.g. even if KVM
> were to redirect to a dummy register, KVM would be incorrectly read zeros
> and drop writes.
> 
> Note, bugging the VM doesn't completely prevent data corruption, e.g. the
> current round of emulation will complete before the vCPU bails out to
> userspace.  But, the very act of killing the guest can also cause data
> corruption, e.g. due to lack of file writeback before termination, so
> taking on additional complexity to cleanly bail out of the emulator isn't
> justified, the goal is purely to stem the bleeding and alert userspace
> that something has gone horribly wrong, i.e. to avoid _silent_ data
> corruption.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

I like this -- this ends up failing in a relatively clean fashion. (i.e.
it's not actually a BUG(), but rather tells the VM to stop.)

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook