Subject: Re: sudo x86info -a => kernel BUG at mm/usercopy.c:78!
To: Linus Torvalds <torvalds@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>
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CC: 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>
From: Tommi Rantala <tommi.t.rantala@nokia.com>
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Date: Fri, 31 Mar 2017 22:32:53 +0300
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On 31.03.2017 21:26, Linus Torvalds wrote:
> Hmm. Thinking more about this, we do allow access to the first 1MB of
> physical memory unconditionally (see devmem_is_allowed() in
> arch/x86/mm/init.c). And I think we only _reserve_ the first 64kB or
> something. So I guess even STRICT_DEVMEM isn't actually all that
> strict.
>
> So this should be visible even *with* STRICT_DEVMEM.
>
> Does a simple
>
>      sudo dd if=/dev/mem of=/dev/null bs=4096 count=256
>
> also show the same issue? Maybe regardless of STRICT_DEVMEM?

Yep, it is enough to trigger the bug.

Also crashes with the fedora kernel that has STRICT_DEVMEM:

$ sudo dd if=/dev/mem of=/dev/null bs=4096 count=256
Segmentation fault

[   73.224025] usercopy: kernel memory exposure attempt detected from 
ffff893a80059000 (dma-kmalloc-16) (4096 bytes)
[   73.224049] ------------[ cut here ]------------
[   73.224056] kernel BUG at mm/usercopy.c:75!
[   73.224060] invalid opcode: 0000 [#1] SMP
[   73.224237] CPU: 5 PID: 2860 Comm: dd Not tainted 
4.9.14-200.fc25.x86_64 #1


> Maybe we should change devmem_is_allowed() to return a ternary value,
> and then have it be "allow access" (for reserved pages), "disallow
> access" (for various random stuff), and "just read zero" (for pages in
> the low 1M that aren't marked reserved).
>
> That way things like that read the low 1M (like x86info) will
> hopefully not be unhappy, but also won't be reading random kernel
> data.
>
>               Linus
>