Re: Updated version of RD/WR FS/GS BASE patchkit

From: Andy Lutomirski
Date: Mon Mar 21 2016 - 18:58:05 EST

On Mon, Mar 21, 2016 at 3:52 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
>> No. My objection is that there needs to be an explicit statement what
>> the semantics are. If the agreed-upon semantics are "undefined
>> behavior if GS != 0 and GSBASE doesn't match the descriptor", so be
>> it, but this needs to be a conscious decision and needs to be weighed
>> against the alternatives.
> Documentation/x86/fsgs.txt already has this statement:
> Another requirement is that the FS or GS selector has to be zero
> (is normally true unless changed explicitly). When it is non-zero
> the context switch assumes the bases were loaded through the LDT/GDT,
> and will reload that.
> <<<
> Is that sufficient?

Maybe. Are there better options? Could we, for example, actually try
to preserve the state if this happens? Would it be worth it?

>> The actual implementation details are just details. They need to
>> match the intended semantics, of course.
> I believe my implementation matches the paragraph above.
> -Andi
> --
> ak@xxxxxxxxxxxxxxx -- Speaking for myself only.

Andy Lutomirski
AMA Capital Management, LLC