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

Andy Lutomirski
2015-02-26T19:17:59Z

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On Thu, Feb 26, 2015 at 8:28 AM, Brian Gerst <brgerst@gmail.com> wrote:
> On Thu, Feb 26, 2015 at 10:32 AM, Andy Lutomirski <luto@amacapital.net> wrote:
>> 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.
>
> 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

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/*
* Copyright (c) 2014-2015 Andy Lutomirski
* GPL v2
*/

#include <stdio.h>
#include <unistd.h>
#include <time.h>
#include <err.h>
#include <asm/ldt.h>
#include <sys/syscall.h>

static unsigned short GDT3(int idx)
{
return (idx << 3) | 3;
}

static int create_tls(int idx, unsigned int base)
{
struct user_desc desc = {
.entry_number = idx,
.base_addr = base,
.limit = 0xfffff,
.seg_32bit = 1,
.contents = 0, /* Data, grow-up */
.read_exec_only = 0,
.limit_in_pages = 1,
.seg_not_present = 0,
.useable = 0,
};

if (syscall(SYS_set_thread_area, &desc) != 0)
err(1, "set_thread_area");

return desc.entry_number;
}

int main()
{
int idx = create_tls(-1, 0);
printf("Allocated GDT index %d\n", idx);

unsigned short orig_es;
asm volatile ("mov %%es,%0" : "=rm" (orig_es));

int errors = 0;
int total = 1000;
for (int i = 0; i < total; i++) {
struct timespec req = {
.tv_sec = 0,
.tv_nsec = 100000,
};
int ret;

asm volatile ("mov %0,%%es" : : "rm" (GDT3(idx)));

/*
* Force rescheduling. On 32-bit kernels, fast syscalls
* destroy DS and ES, so force int 80.
*/
asm volatile ("int $0x80"
: "=a" (ret)
: "a" (SYS_nanosleep), "b" (&req),
"c" (0));

unsigned short es;
asm volatile ("mov %%es,%0" : "=rm" (es));
asm volatile ("mov %0,%%es" : : "rm" (orig_es));
if (es != GDT3(idx)) {
if (errors == 0)
printf("[FAIL]\tES changed from 0x%hx to 0x%hx\n",
GDT3(idx), es);
errors++;
}
}

if (errors) {
printf("[FAIL]\tES was corrupted %d/%d times\n", errors, total);
return 1;
} else {
printf("[OK]\tES was preserved\n");
return 0;
}
}