linux-kernel - Re: sudo x86info -a => kernel BUG at mm/usercopy.c:78!

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Date:   Tue, 4 Apr 2017 17:22:27 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Tommi Rantala <tommi.t.rantala@...ia.com>,
        Dave Jones <davej@...emonkey.org.uk>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Laura Abbott <labbott@...hat.com>,
        Ingo Molnar <mingo@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Eric Biggers <ebiggers@...gle.com>
Subject: Re: sudo x86info -a => kernel BUG at mm/usercopy.c:78!

On Tue, Apr 4, 2017 at 3:55 PM, Linus Torvalds
<torvalds@...ux-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