From mboxrd@z Thu Jan  1 00:00:00 1970
From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Subject: Re: [PATCH v2 2/2] x86/hvm: Don't intercept #UD
	exceptions in general
Date: Wed, 27 Jan 2016 14:26:14 -0500
Message-ID: <56A919D6.3080703@oracle.com>
References: <56A91122.50502@oracle.com>
	<1453921984-29197-1-git-send-email-andrew.cooper3@citrix.com>
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To: Andrew Cooper <andrew.cooper3@citrix.com>, Xen-devel <xen-devel@lists.xen.org>
Cc: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>, Kevin Tian <kevin.tian@intel.com>, Aravind Gopalakrishnan <Aravind.Gopalakrishnan@amd.com>, Jun Nakajima <jun.nakajima@intel.com>, Jan Beulich <JBeulich@suse.com>
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On 01/27/2016 02:13 PM, Andrew Cooper wrote:
> c/s 0f1cb96e "x86 hvm: Allow cross-vendor migration" caused HVM domains to
> unconditionally intercept #UD exceptions.  While cross-vendor migration is
> cool as a demo, it is extremely niche.
>
> Intercepting #UD allows userspace code in a multi-vcpu guest to execute
> arbitrary instructions in the x86 emulator by having one thread execute a ud2a
> instruction, and having a second thread rewrite the instruction before the
> emulator performs an instruction fetch.
>
> XSAs 105, 106 and 110 are all examples where guest userspace can use bugs in
> the x86 emulator to compromise security of the domain, either by privilege
> escalation or causing a crash.
>
> c/s 2d67a7a4 "x86: synchronize PCI config space access decoding"
> introduced (amongst other things) a per-domain vendor, based on the guests
> cpuid policy.
>
> Use the per-guest vendor to enable #UD interception only when a domain is
> configured for a vendor different to the current hardware.  (#UD interception
> is also enabled if hvm_fep is specified on the Xen command line.  This is a
> debug-only option whose entire purpose is for testing the x86 emulator.)
>
> As a result, the overwhelming majority of usecases now have #UD interception
> disabled, removing an attack surface for malicious guest userspace.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>


Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>