[patch] restrict amd64_set_{f,g}sbase() to values inside user VA

[David Xu]

I had already post similar patch some days ago to re@

David Xu

Joseph Koshy wrote:

>Does anyone have objections to the following patch?
>
>It restricts the values that can be loaded into the FS.base and
>GS.base MSRs for user processes to those inside of user VA 
>(0..(1<<47) today).
>
>These values are used as the base addresses for FS- and GS-
>relative addressing, when a FS: or GS: segment override
>is specified in an instruction.
>  
>