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

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Date:   Thu, 30 Mar 2017 18:20:12 +0100
From:   Mark Rutland <mark.rutland@....com>
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>,
        Eric Biggers <ebiggers@...gle.com>,
        Dave Jones <davej@...emonkey.org.uk>
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.
> 
> > [...]
> > [   51.419063] Call Trace:
> > [   51.419066]  read_mem+0x70/0x120
> > [   51.419069]  __vfs_read+0x28/0x130
> > [   51.419072]  ? security_file_permission+0x9b/0xb0
> > [   51.419075]  ? rw_verify_area+0x4e/0xb0
> > [   51.419077]  vfs_read+0x96/0x130
> > [   51.419079]  SyS_read+0x46/0xb0
> > [   51.419082]  ? SyS_lseek+0x87/0xb0
> > [   51.419085]  entry_SYSCALL_64_fastpath+0x1a/0xa9
> 
> I can't reproduce this myself, so I assume it's some specific /proc or
> /sys file that I don't have. Are you able to get a strace of x86info
> as it runs to see which file it is attempting to read here?

Presumably this is /dev/mem, with read_mem in drivers/char/mem.c.

I guess you may have locked that down on your system anyhow. ;)

Thanks,
Mark.