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

From: Laura Abbott
Date: Thu Mar 30 2017 - 13:44:27 EST


On 03/30/2017 10:37 AM, Kees Cook wrote:
> On Thu, Mar 30, 2017 at 10:27 AM, Laura Abbott <labbott@xxxxxxxxxx> 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@xxxxxxxxx> 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
>

CONFIG_STRICT_DEVMEM should be on in all Fedora kernels.

Thanks,
Laura