linux-kernel - Re: [PATCH] KVM: x86: restore IP after all far jump failures

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Date:   Tue, 22 Nov 2016 21:56:01 +0100
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>, stable@...r.kernel.org,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Steve Rutherford <srutherford@...gle.com>,
        Andrew Honig <ahonig@...gle.com>,
        Prasad Pandit <ppandit@...hat.com>
Subject: Re: [PATCH] KVM: x86: restore IP after all far jump failures

2016-11-22 11:43-0800, Nadav Amit:
> I admit my wrongdoings, but I still think the fix should have been to
> remove the entire recovery logic and just return X86EMUL_UNHANDLEABLE if
> something goes wrong (exception). This will kill the misbehaving process
> but keep the VM running.

X86EMUL_UNHANDLEABLE will kill the whole VM (on QEMU, other userspaces
might handle the instruction and resume KVM).

The recovery path is in the spec, which means that nothing goes wrong.
I think we implement the spec quite well now, so keeping the #GP and CS
recovery is slightly better, although not safer.

> Otherwise, a malicious VM process, which can somehow control descriptors
> (LDT?) may modify the descriptor during the emulation and get the system
> to inconsistent state and prevent the VM-entry.

We restore the original CS -- malicious guest would get killed on a
failed VM entry anyway, so the difference is only in KVM internal error
code (assuming there are no other bugs).

Am I misunderstanding something?