linux-kernel - Re: [PATCH 06/11] x86-64: Fix %cs value in convert_from

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Message-ID: <AANLkTinVQFHG1nh5XBd1tJ29fBDjQc=QRThXySiPWd2q@mail.gmail.com>
Date:	Sun, 29 Aug 2010 20:25:16 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Pekka Enberg <penberg@...nel.org>
Cc:	hpa@...or.com, x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 06/11] x86-64: Fix %cs value in convert_from_fxsr()

On Sun, Aug 29, 2010 at 2:41 PM, Pekka Enberg <penberg@...nel.org> wrote:
> On Sat, Aug 28, 2010 at 7:04 PM, Brian Gerst <brgerst@...il.com> wrote:
>> While %ds still contains the userspace selector, %cs is KERNEL_CS
>> at this point.  Always get %cs from pt_regs.
>>
>> It actually is possible to get the correct segments for compat tasks,
>> but that involves using the [f]xsave instruction without a REX.W prefix.
>>
>> Signed-off-by: Brian Gerst <brgerst@...il.com>
>
> It might be just me but the above description doesn't explain
> anything. What's the problem here? What is this fixing?

The %cs segment being reported to a compat task is flat out wrong.  It
is getting KERNEL_CS when it should be some userspace segment.  The
code segment may still be wrong, because the %cs in pt_regs may not
have been the segment where the instruction that flagged the exception
executed from.  That could be fixed by using fxsave without a REX.W
prefix when saving the state of compat tasks, which would save the
segment and 32-bit offset instead of the 64-bit offset for the code
and data pointers.  This is such a corner case that it probably isn't
worth putting much effort into fixing unless someone demonstrates a
real need for it.

--
Brian Gerst
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