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Re: [Xen-devel] fxrstor recovery code

To: "Jan Beulich" <jbeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] fxrstor recovery code
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Mon, 24 Apr 2006 15:58:51 +0100
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On 24 Apr 2006, at 14:54, Jan Beulich wrote:

Since native Linux isn't considering the possibility of fxrstor faulting because of bad data I'm wondering why Xen is, and what kind of fault it is being thought of here; after all, the documentation also doesn't say anything like that.

Linux only FXRSTORs state that was previously saved by the kernel using FXSAVE. Hence Linux knows that the data is valid and reloading it will not fault.

This is not the case in Xen, where the FPU info may come from a saved image file (if someone executes 'xm restore' on an image file). The FPU data block may contain bogus or malicious data and Xen must protect itself from that.

Concretely, if the info has a corrupted MXCSR with 1s in reserved bit positions then FXRSTOR will cause a general-protection fault. This isn't listed in the Protected Mode Exceptions section of the Intel reference manual, but see the last sentence in the main description for the instruction.

 -- Keir

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