linux-kernel - Re: [PATCH] KVM: X86: Fix the decoding of segment overrides in 64bit mode

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Message-ID: <CANRm+CwsFX7rVkPvENRM20D84zotonxFFJMSq98icS4BQm7o2Q@mail.gmail.com>
Date:   Fri, 23 Mar 2018 22:27:57 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH] KVM: X86: Fix the decoding of segment overrides in 64bit mode

2018-03-22 21:53 GMT+08:00 Andrew Cooper <andrew.cooper3@...rix.com>:
> On 22/03/18 13:39, Wanpeng Li wrote:
>> 2018-03-22 20:38 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
>>> On 22/03/2018 12:04, Andrew Cooper wrote:
>>>> We've got a Force Emulation Prefix (ud2a; .ascii "xen") for doing
>>>> magic.  Originally, this was used for PV guests to explicitly request an
>>>> emulated CPUID, but I extended it to HVM guests for "emulate the next
>>>> instruction", after we had some guest user => guest kernel privilege
>>>> escalations because of incorrect emulation.
>>> Wanpeng, why don't you add it behind a new kvm module parameter? :)
>> Great point! I will have a try. Thanks Paolo and Andrew. :)
>
> Using the force emulation prefix requires intercepting #UD, which is in
> general a BadThing(tm) for security.  Therefore, we have a build time

Yeah, however kvm intercepts and emulates #UD by default, should we
add a new kvm module parameter to enable it and disable by default?
Paolo.

> configuration option to compile in support, and require that test
> systems explicitly opt into using it via a command line parameter.
>
> http://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=xen/arch/x86/hvm/hvm.c;h=db52312882205e65b32e587106ca795f4bfab2eb;hb=refs/heads/staging#l3741
> is the general #UD intercept handler if you want a reference.  (You can

Thanks Andrew, it is useful. :) In addition, I didn't see the
test-memop-seg testcase has "Forced Emulation Prefix", when the prefix
is added to each instruction in the testcase?

Regards,
Wanpeng Li