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

Brian Gerst
2015-02-25T03:23:54Z

On Tue, Feb 24, 2015 at 3:08 PM, Denys Vlasenko
<vda.linux@googlemail.com> wrote:
> On Tue, Feb 24, 2015 at 9:02 PM, Andy Lutomirski <luto@amacapital.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.

--
Brian Gerst