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

[Andy Lutomirski]

On Tue, Mar 21, 2017 at 5:41 PM, Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
> 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).
>

Talk about barking up the wrong tree...

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index f8e22dbad86c..c564f62c7a8d 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -462,7 +464,8 @@ pgprot_t pg_fixmap_gdt_flags = PAGE_KERNEL;
 static inline void setup_fixmap_gdt(int cpu)
 {
        __set_fixmap(get_cpu_gdt_ro_index(cpu),
-                    __pa(get_cpu_gdt_rw(cpu)), pg_fixmap_gdt_flags);
+                    slow_virt_to_phys(get_cpu_gdt_rw(cpu)),
+                    pg_fixmap_gdt_flags);
 }

 /* Load the original GDT from the per-cpu structure */

This makes UP boot for me, but SMP (2 cpus) is still busted.