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From: Andy Lutomirski <luto@amacapital.net>
Date: Fri, 17 Mar 2017 13:18:23 -0700
Message-ID: <CALCETrU328Y-N7BEOCp_5KVzW0beGoEUBk+2PzLpFXYjvaKV4Q@mail.gmail.com>
Subject: Re: [x86] 45fc8757d1: BUG:unable_to_handle_kernel
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>,
        kernel test robot <xiaolong.ye@intel.com>,
        Thomas Garnier <thgarnie@google.com>, Ingo Molnar <mingo@kernel.org>,
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On Fri, Mar 17, 2017 at 12:36 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Fri, Mar 17, 2017 at 11:20 AM, Andy Lutomirski <luto@kernel.org> wrote:
>>
>> I can easily imagine that breaking WINE or DOSEMU because it'll affect
>> the LDT, too.
>
> 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.)

>
>> How about this:
>
> I don't think that's _wrong_, but..
>
> I'd really rather just do it in fill_ldt() itself, unless you can
> explain how it would be visible to anybody..

See above :(

(Also, your approach would probably break some selftests.)

--Andy