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

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Date:   Thu, 30 Mar 2017 10:37:58 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Laura Abbott <labbott@...hat.com>
Cc:     Tommi Rantala <tommi.t.rantala@...ia.com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Mark Rutland <mark.rutland@....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 10:27 AM, Laura Abbott <labbott@...hat.com> wrote:
> On 03/30/2017 09:45 AM, 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?
>
> I can't see this on any of my Fedora systems. It looks like this
> is trying to read /dev/mem so I suspect your BIOS is putting out
> unexpected values. If you turn off hardened usercopy does x86info
> give you reasonable values? I'd also echo getting an strace.

Reads out of /dev/mem should be restricted to non-RAM on Fedora, yes?

Tommi, do your kernels have CONFIG_STRICT_DEVMEM=y ?

-Kees

-- 
Kees Cook
Pixel Security