linux-kernel - Re: [lkp-robot] [x86] 69218e4799: BUG:kernel_hang_in_boot

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Date:   Tue, 21 Mar 2017 14:11:25 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Thomas Garnier <thgarnie@...gle.com>
Cc:     kernel test robot <xiaolong.ye@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        Alexander Potapenko <glider@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Borislav Petkov <bp@...e.de>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Jiri Kosina <jikos@...nel.org>, Joerg Roedel <joro@...tes.org>,
        Jonathan Corbet <corbet@....net>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Kees Cook <keescook@...omium.org>,
        Len Brown <len.brown@...el.com>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        "Luis R . Rodriguez" <mcgrof@...nel.org>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Michal Hocko <mhocko@...e.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Rusty Russell <rusty@...tcorp.com.au>,
        Stanislaw Gruszka <sgruszka@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        zijun_hu <zijun_hu@....com>, LKML <linux-kernel@...r.kernel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>, LKP <lkp@...org>
Subject: Re: [lkp-robot] [x86] 69218e4799: BUG:kernel_hang_in_boot_stage

On Tue, Mar 21, 2017 at 1:25 PM, Thomas Garnier <thgarnie@...gle.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