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

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Date:   Thu, 22 Mar 2018 13:53:25 +0000
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Wanpeng Li <kernellwp@...il.com>,
        Paolo Bonzini <pbonzini@...hat.com>
CC:     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

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
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
ignore the cross-vendor part, which is leftovers from
http://developer.amd.com/wordpress/media/2012/10/CrossVendorMigration.pdf )

~Andrew