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

[Andy Lutomirski]

On Tue, Feb 24, 2015 at 7:47 AM, Denys Vlasenko
<vda.linux@xxxxxxxxxxxxxx> wrote:
> On Tue, Feb 17, 2015 at 2:46 AM, Ben Hutchings <ben@xxxxxxxxxxxxxxx> wrote:
>> 3.2.67-rc1 review patch.  If anyone has any objections, please let me know.
>>
>> ------------------
>>
>> From: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
>>
>> commit f647d7c155f069c1a068030255c300663516420e upstream.
>>
>> Otherwise, if buggy user code points DS or ES into the TLS
>> array, they would be corrupted after a context switch.
>>
>> This also significantly improves the comments and documents some
>> gotchas in the code.
>>
>> Before this patch, the both tests below failed.  With this
>> patch, the es test passes, although the gsbase test still fails.
>>
>>  ----- begin es test -----
>>
>> /*
>>  * Copyright (c) 2014 Andy Lutomirski
>>  * GPL v2
>>  */
>>
>> 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++) {
>>                 asm volatile ("mov %0,%%es" : : "rm" (GDT3(idx)));
>>                 usleep(100);
>>
>>                 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;
>>         }
>> }
>>
>>  ----- end es test -----
>
> 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.

--Andy
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