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In-Reply-To: <20170317115924.GA82960@inn.lkp.intel.com>
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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Fri, 17 Mar 2017 10:49:47 -0700
Message-ID: <CA+55aFwx5D1hSaW6ac3D=30cY7=2i4cPmGn9zB-1a8ywpau3cw@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>,
        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>,
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        Pavel Machek <pavel@ucw.cz>, Peter Zijlstra <peterz@infradead.org>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
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        Thomas Gleixner <tglx@linutronix.de>,
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On Fri, Mar 17, 2017 at 4:59 AM, kernel test robot
<xiaolong.ye@intel.com> wrote:
>
> FYI, we noticed the following commit:
>
> commit: 45fc8757d1d2128e342b4e7ef39adedf7752faac ("x86: Make the GDT remapping read-only on 64-bit")
> https://git.kernel.org/cgit/linux/kernel/git/tip/tip.git x86/mm
>
> in testcase: boot
>
> [    4.347219] BUG: unable to handle kernel paging request at ffffffffff577060
> [    4.360480] Oops: 0003 [#1] SMP
> [    4.373550] RIP: 0023:0xf77e91ed
> [    4.375284] RSP: 002b:00000000ffed034c EFLAGS: 00010246

Heh. That's actually in user space, but the error code (0003) means
"protection fault on a write, not a user access".

So it's almost certainly something that tries to access a segment
descriptor in the GDT, but that segment was marked as "not accessed",
and the CPU was trying to set the accessed bit.

I *thought* we always maked everything accessed when we initialize it,
but something clearly is not.

That's why there's no kernel call trace or anything like that: it is a
system page fault, but it's triggered directly from user mode.

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..

             Linus