linux-kernel - Re: [PATCH 3.2 055/152] x86_64, switch_to(): Load TLS descriptors before switching DS and ES

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Date:	Thu, 26 Feb 2015 11:17:59 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Denys Vlasenko <vda.linux@...glemail.com>,
	Ben Hutchings <ben@...adent.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	stable <stable@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andi Kleen <andi@...stfloor.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 3.2 055/152] x86_64, switch_to(): Load TLS descriptors
 before switching DS and ES

On Thu, Feb 26, 2015 at 8:28 AM, Brian Gerst <brgerst@...il.com> wrote:
> On Thu, Feb 26, 2015 at 10:32 AM, Andy Lutomirski <luto@...capital.net> wrote:
>> On Tue, Feb 24, 2015 at 7:23 PM, Brian Gerst <brgerst@...il.com> wrote:
>>> On Tue, Feb 24, 2015 at 3:08 PM, Denys Vlasenko
>>> <vda.linux@...glemail.com> wrote:
>>>> On Tue, Feb 24, 2015 at 9:02 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>>>>> This currently fails in 32-bit kernels (at least in qemu):
>>>>>>
>>>>>> / # ./es_test
>>>>>> Allocated GDT index 7
>>>>>> [FAIL]    ES changed from 0x3b to 0x7b
>>>>>> [FAIL]    ES was corrupted 1000/1000 times
>>>>>> / # uname -a
>>>>>> Linux (none) 4.0.0-rc1 #1 SMP Tue Feb 24 16:41:58 CET 2015 i686 GNU/Linux
>>>>>
>>>>> Want to send a patch?  I'll get it in a few days if no one beats me.
>>>>
>>>> I have no patch, sorry (in fact, I failed to find where is the relevant
>>>> 32-bit counterpart).
>>>>
>>>> It's just security people asked me to backport this and I wondered
>>>> maybe I should wait a bit on this one, since fix for 32-bit ought
>>>> to appear as well.
>>>
>>> For 32-bit kernel, userspace DS and ES are saved at syscall/interrupt
>>> entry time and reloaded on exit, unlike in 64-bit where they are saved
>>> and loaded at context switch time.  Therefore 32-bit is not affected
>>> by the issue this patch addresses.
>>>
>>> It looks to me though, that the ES test program doesn't actually test
>>> what the patch fixes - the segment attributes, like the base address.
>>> It tests just the selector, which shouldn't change across a kernel
>>> entry (with a few exceptions, like signals).  If the test is failing,
>>> then it is a different issue from what this patch addresses.
>>
>> It tests it indirectly.  The 64-bit code sets the selector to zero if
>> it fails to reload it.  Testing the ES base is awkward because it
>> can't be done in 64-bit code at all.
>
> I figured out why Denys got the failure.  usleep() makes a syscall via
> sysenter.  The sysenter path saves es/ds, but does not restore them
> before sysexit like the int80/iret path would.  That leaves them as
> USER_DS that the kernel loaded for itself.  I believe this was an
> intentional optimization, assuming the vdso would only be called from
> programs conforming to the ELF ABI.

Makes sense.  The attached variant passes, so I think we're fine.

--Andy

View attachment "estest.c" of type "text/x-csrc" (1696 bytes)