linux-kernel - Re: [PATCH 4/7] x86: Add support for rd/wr fs/gs base

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Message-ID: <5362CE87.7010908@zytor.com>
Date:	Thu, 01 May 2014 15:45:27 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andi Kleen <ak@...ux.intel.com>
CC:	Andi Kleen <andi@...stfloor.org>,
	Andy Lutomirski <luto@...capital.net>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/7] x86: Add support for rd/wr fs/gs base

On 05/01/2014 03:18 PM, Andi Kleen wrote:
>> I haven't looked through the flows (I'm at LCE so I have limited screen
>> bandwidth) to see how that would be handled in this case, but in the
>> general paranoid case it comes down to the fact that in this particular
>> subcase we don't necessarily know exactly how many SWAPGS are between us
>> and userspace after we IRET.
> 
> There is none as far as I know. Certainly wasn't any when the code
> was originally written.
> 

This applies for an asynchronous entry from kernel space.  Obviously in
the case where we actually come directly from user space (the stack
frame CS.RPL == 3) then that doesn't apply.

	-hpa


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