Messages in this thread | | | From | Andy Lutomirski <> | Date | Tue, 21 Mar 2017 16:51:21 -0700 | Subject | Re: [lkp-robot] [x86] 69218e4799: BUG:kernel_hang_in_boot_stage |
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On Tue, Mar 21, 2017 at 3:32 PM, Andy Lutomirski <luto@amacapital.net> wrote: > On Tue, Mar 21, 2017 at 2:11 PM, Linus Torvalds > <torvalds@linux-foundation.org> wrote: >> On Tue, Mar 21, 2017 at 1:25 PM, Thomas Garnier <thgarnie@google.com> 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?
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