Re: sudo x86info -a => kernel BUG at mm/usercopy.c:78!
From: Kees Cook
Date: Thu Mar 30 2017 - 13:38:06 EST
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
--
Kees Cook
Pixel Security