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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Tue, 21 Mar 2017 14:11:25 -0700
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Subject: Re: [lkp-robot] [x86] 69218e4799: BUG:kernel_hang_in_boot_stage
To: Thomas Garnier <thgarnie@google.com>
Cc: kernel test robot <xiaolong.ye@intel.com>,
        Ingo Molnar <mingo@kernel.org>,
        Alexander Potapenko <glider@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Matt Fleming <matt@codeblueprint.co.uk>,
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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.

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.

              Linus