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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Tue, 4 Apr 2017 17:22:27 -0700
Message-ID: <CA+55aFwv8QPBD4SMLw2Y7qkV4JceMc9NdOujbVM7PfcBpkhm3Q@mail.gmail.com>
Subject: Re: sudo x86info -a => kernel BUG at mm/usercopy.c:78!
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To: Kees Cook <keescook@chromium.org>
Cc: Tommi Rantala <tommi.t.rantala@nokia.com>, Dave Jones <davej@codemonkey.org.uk>, Linux-MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>, Laura Abbott <labbott@redhat.com>, Ingo Molnar <mingo@kernel.org>, Josh Poimboeuf <jpoimboe@redhat.com>, Mark Rutland <mark.rutland@arm.com>, Eric Biggers <ebiggers@google.com>

On Tue, Apr 4, 2017 at 3:55 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> I already explained what the likely fix is: make devmem_is_allowed()
> return a ternary value, so that those things that *do* read the BIOS
> area can just continue to do so, but they see zeroes for the parts
> that the kernel has taken over.

Actually, a simpler solution might be to

 (a) keep the binary value

 (b) remove the test for the low 1M

 (c) to avoid breakage, don't return _error_, but just always read zero

that also removes (or at least makes it much more expensive) a signal
of which pages are kernel allocated vs BIOS allocated.

               Linus

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