linux-kernel - Re: [PATCH 4/8] x86: Add support for rd/wr fs/gs base

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Date:	Tue, 11 Nov 2014 12:49:16 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Andi Kleen <andi@...stfloor.org>, X86 ML <x86@...nel.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...ux.intel.com>, Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 4/8] x86: Add support for rd/wr fs/gs base

On Tue, Nov 11, 2014 at 12:05 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On 11/10/2014 03:55 PM, Andi Kleen wrote:
>> To prevent recursive interrupts clobbering this
>> state in the task_struct this is only done for interrupts
>> coming directly from ring 3.
>
> Since this just came up in a different context today, I'd like to
> propose a different solution to this piece of the problem.
>
> Can we change the paranoid entry to check if the entry came from ring 3
> and to just switch stacks immediately to the standard kernel stack and
> run the non-paranoid entry code?  This eliminates paranoid_userspace
> entirely, and there are no special gsbase machinations any more for the
> entry-from-userspace path.
>
> In fact, I think that this will result in the MSR KERNEL_GS_BASE value
> *always* matching the userspace gs value from any C code in the kernel,
> since we'll always swapgs exactly once on entry from userspace.

I'll send patches in a couple hours.  I have it mostly working.

--Andy
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