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

[Denys Vlasenko]

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
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