Re: [Xen-devel] [PATCH v3 2/2] x86/hvm: Don't intercept #UD exceptions in general

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
> Sent: Saturday, January 30, 2016 3:18 AM
> 
> 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>

Acked-by: Kevin Tian <kevin.tian@xxxxxxxxx>

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