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

Andy Lutomirski
2015-02-26T15:32:12Z

On Tue, Feb 24, 2015 at 7:23 PM, Brian Gerst <brgerst@gmail.com> wrote:
> 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.

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.

--Andy