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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Fri, 17 Mar 2017 11:07:48 -0700
Message-ID: <CA+55aFzYxqhLoDcepjt4Z3uLRBNdAg5Ou25Uj9vzZO2RfaVrDQ@mail.gmail.com>
Subject: Re: [x86] 45fc8757d1: BUG:unable_to_handle_kernel
To: kernel test robot <xiaolong.ye@intel.com>
Cc: Thomas Garnier <thgarnie@google.com>, Ingo Molnar <mingo@kernel.org>,
        Alexander Potapenko <glider@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Andy Lutomirski <luto@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
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        Christian Borntraeger <borntraeger@de.ibm.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Jiri Kosina <jikos@kernel.org>, Joerg Roedel <joro@8bytes.org>,
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        Juergen Gross <jgross@suse.com>, Kees Cook <keescook@chromium.org>,
        Len Brown <len.brown@intel.com>, Lorenzo Stoakes <lstoakes@gmail.com>,
        "Luis R . Rodriguez" <mcgrof@kernel.org>,
        Matt Fleming <matt@codeblueprint.co.uk>,
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        Thomas Gleixner <tglx@linutronix.de>,
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On Fri, Mar 17, 2017 at 11:00 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Fri, Mar 17, 2017 at 10:49 AM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>>
>> The linear address can be used to look up which entry it is. I assume
>> the GDT starts at ffffffffff577000, and that this is at offset 0x60
>> from that. Whatever descriptor that would be..
>
> Hmm. That should be gdt index 12, aka GDT_ENTRY_TLS_MIN.
>
> I guess user space can set almost anything there. Including setting a
> segment type that isn't accessed, and that the CPU will change on the
> first actual access.
>
> We do have code to verify the limits and types etc iirc, I guess we
> can make sure to set the accessed bit too.

Hmm. "fill_ldt()" does this:

        desc->type              = (info->read_exec_only ^ 1) << 1;
        desc->type             |= info->contents << 2;

which always leaves bit #0 of ->type clear. That's the A bit.

Does the problem go away if we just add a

        desc->type             |= 1;

to the end there?

But it is entirely possible that I'm missing something here. It's been
_years_ since I looked at descriptor table entries.

                Linus