linux-kernel - Re: [PATCH 7/8] x86: Add documentation for rd/wr fs/gs base

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Date:	Fri, 10 Apr 2015 13:38:14 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	Andi Kleen <andi@...stfloor.org>, X86 ML <x86@...nel.org>,
	Andrew Lutomirski <luto@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 7/8] x86: Add documentation for rd/wr fs/gs base

On Fri, Apr 10, 2015 at 1:22 PM, Andi Kleen <ak@...ux.intel.com> wrote:
>
>
>
>> I think that your __switch_to is buggy and that's why the selectors
>> need to be zero.  Is that the only issue?  If so, let's fix the bug
>> instead.
>
> I don't think there is a bug.

So what's the issue?

If user code programs fs != 0 and fsbase != whatever is implied by fs
and the GDT/LDT, what happens?  There's already a minor buglet in that
area without wrfsbase, but fixing it would be a big performance hit
because we don't have rdfsbase and rdgsbase to read the state
efficiently.  (Specifically, if we have gs == 0, gsbase == 0, but
*saved* gsbase != 0, then we corrupt gsbase on context switch.)

But, with the new instructions, we can do it simply, efficiently, and
correctly in all cases.  Let's do so.

--Andy

>
> -Andi
>
> --
> ak@...ux.intel.com -- Speaking for myself only

-- 
Andy Lutomirski
AMA Capital Management, LLC
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