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

Andy Lutomirski
2015-02-24T20:02:25Z

On Tue, Feb 24, 2015 at 7:47 AM, Denys Vlasenko
<vda.linux@googlemail.com> wrote:
> On Tue, Feb 17, 2015 at 2:46 AM, Ben Hutchings <ben@decadent.org.uk> wrote:
>> 3.2.67-rc1 review patch. If anyone has any objections, please let me know.
>>
>> ------------------
>>
>> From: Andy Lutomirski <luto@amacapital.net>
>>
>> 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