Re: [lkp-robot] [x86] 69218e4799: BUG:kernel_hang_in_boot_stage

[Thomas Garnier]

On Tue, Mar 21, 2017 at 4:51 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> On Tue, Mar 21, 2017 at 3:32 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> On Tue, Mar 21, 2017 at 2:11 PM, Linus Torvalds
>> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>>> On Tue, Mar 21, 2017 at 1:25 PM, Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
>>>> The issue seems to be related to exceptions happening in close pages
>>>> to the fixmap GDT remapping.
>>>>
>>>> The original page fault happen in do_test_wp_bit which set a fixmap
>>>> entry to test WP flag. If I grow the number of processors supported
>>>> increasing the distance between the remapped GDT page and the WP test
>>>> page, the error does not reproduce.
>>>>
>>>> I am still looking at the exact distance between repro and no-repro as
>>>> well as the exact root cause.
>>>
>>> Hmm. Have we set the GDT limit incorrectly, somehow? The GDT *can*
>>> cover 8k entries, which at 8 bytes each would be 64kB.
>>
>> The QEMU barf says the GDT limit is 0xff, for better or for worse.
>>
>>>
>>> So somebody trying to load an invalid segment (say, 0xffff) might end
>>> up causing an access to the GDT base + 64k - 8.
>>>
>>> It is also possible that the CPU might do a page table writability
>>> check *before* it does the limit check. That would sound odd, though.
>>> Might be a CPU errata.
>>>
>>
>
>> There's presumably something genuinely wrong with our GDT.
>
> This is suspicious.  I added this code in test_wp_bit:
>
>     if (memcmp(get_current_gdt_ro(), get_current_gdt_rw(), 4096) != 0) {
>         pr_err("Oh crap\n");
>         BUG_ON(1);
>     }
>
> It printed "Oh crap" and blew up.  Methinks something's wrong with the
> fixmap.  Is it possible that we're crossing a PMD boundary and failing
> to translate the addresses right?

I might be that. We crash when the PKMAP_BASE is just after the FIX_WP_TEST.

I will continue testing couple scenarios and design a fix. Moving the
GDT FIXMAP at the beginning or align the base (or pad the end).

-- 
Thomas