Re: [Xen-devel] fxrstor recovery code

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

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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