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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Fri, 17 Mar 2017 14:10:42 -0700
Message-ID: <CA+55aFxhsPLddZPaReCa7pUWZfX3vfTTDcUDfZMz3zQ_+rObfA@mail.gmail.com>
Subject: Re: [x86] 45fc8757d1: BUG:unable_to_handle_kernel
To: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>,
        kernel test robot <xiaolong.ye@intel.com>,
        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>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Borislav Petkov <bp@suse.de>, Chris Wilson <chris@chris-wilson.co.uk>,
        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>,
        Jonathan Corbet <corbet@lwn.net>, Josh Poimboeuf <jpoimboe@redhat.com>,
        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>,
        Michal Hocko <mhocko@suse.com>, Paolo Bonzini <pbonzini@redhat.com>,
        Paul Gortmaker <paul.gortmaker@windriver.com>,
        Pavel Machek <pavel@ucw.cz>, Peter Zijlstra <peterz@infradead.org>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        "Rafael J . Wysocki" <rjw@rjwysocki.net>,
        Rusty Russell <rusty@rustcorp.com.au>,
        Stanislaw Gruszka <sgruszka@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>, zijun_hu <zijun_hu@htc.com>,
        LKML <linux-kernel@vger.kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        tipbuild@zytor.com, LKP <lkp@01.org>
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On Fri, Mar 17, 2017 at 1:18 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Fri, Mar 17, 2017 at 12:36 PM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>>
>> Can they even *read* the LDT contents, though? The whole accessed bit
>> doesn't show up in 'struct user_desc', so you can neither set it nor
>> read it.
>
> LAR.  I've learned to never underestimate the absurdity of the games
> played by 16-bit apps.  (See, for example, the fact that some of them
> apparently use SGDT just to find a page that's guaranteed not to be
> accessible.)

Ugh. Right you are, LAR will return those type bits.

Of course, maybe somebody cares about them in the GDT already?  So
it's visible even with your patch, isn't it. We give users four
entries to play with...

                Linus