linux-kernel - Re: [x86] 45fc8757d1: BUG:unable_to_handle

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Date:   Fri, 17 Mar 2017 11:20:51 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     kernel test robot <xiaolong.ye@...el.com>,
        Thomas Garnier <thgarnie@...gle.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>,
        "H. Peter Anvin" <hpa@...or.com>, tipbuild@...or.com,
        LKP <lkp@...org>
Subject: Re: [x86] 45fc8757d1: BUG:unable_to_handle_kernel

On Fri, Mar 17, 2017 at 11:07 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Fri, Mar 17, 2017 at 11:00 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>> On Fri, Mar 17, 2017 at 10:49 AM, Linus Torvalds
>> <torvalds@...ux-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?

I can easily imagine that breaking WINE or DOSEMU because it'll affect
the LDT, too.

How about this:

https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/fixes&id=df8110544c6e899897e1b2ec3ab53d9e4ee40f65

I'll see why selftests didn't catch this, too.