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

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Date:   Thu, 30 Mar 2017 15:41:43 -0400
From:   Dave Jones <davej@...emonkey.org.uk>
To:     Kees Cook <keescook@...omium.org>
Cc:     Tommi Rantala <tommi.t.rantala@...ia.com>,
        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 Thu, Mar 30, 2017 at 09:45:26AM -0700, Kees Cook wrote:
 > On Wed, Mar 29, 2017 at 11:44 PM, Tommi Rantala
 > <tommi.t.rantala@...ia.com> wrote:
 > > Hi,
 > >
 > > Running:
 > >
 > >   $ sudo x86info -a
 > >
 > > On this HP ZBook 15 G3 laptop kills the x86info process with segfault and
 > > produces the following kernel BUG.
 > >
 > >   $ git describe
 > >   v4.11-rc4-40-gfe82203
 > >
 > > It is also reproducible with the fedora kernel: 4.9.14-200.fc25.x86_64
 > >
 > > Full dmesg output here: https://pastebin.com/raw/Kur2mpZq
 > >
 > > [   51.418954] usercopy: kernel memory exposure attempt detected from
 > > ffff880000090000 (dma-kmalloc-256) (4096 bytes)
 > 
 > This seems like a real exposure: the copy is attempting to read 4096
 > bytes from a 256 byte object.

The code[1] is doing a 4k read from /dev/mem in the range 0x90000 -> 0xa0000
According to arch/x86/mm/init.c:devmem_is_allowed, that's still valid..

Note that the printk is using the direct mapping address. Is that what's
being passed down to devmem_is_allowed now ? If so, that's probably what broke.

	Dave

[1] https://github.com/kernelslacker/x86info/blob/master/mptable.c