Re: [Xen-devel] [PATCH v2 2/2] x86/hvm: Don't intercept #UD exceptions in general

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

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@xxxxxxxxxx>

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


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