Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request at 0000a7cf

[Kees Cook]

On Wed, Mar 8, 2017 at 3:55 PM, Laura Abbott <labbott@xxxxxxxxxx> wrote:
> On 03/08/2017 02:36 PM, Kees Cook wrote:
>> On Wed, Mar 8, 2017 at 2:27 PM, Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
>>> [   28.474232] rodata_test: test data was not read only
>>> [...]
>>
>> In my tests so far, I've never been able to get rodata_test to fail
>> (Qemu 2.5.0, Ubuntu). I'll retry with your .config and see if I can
>> recheck under Qemu 2.7.1. Do you see these failures on real hardware?
>>
>> -Kees
>>
>
> FWIW, I'm seeing the same issue with qemu 2.6.2 and 2.8.0 on Fedora 24
> and rawhide respectively.
>
> I also notice that CONFIG_X86_PAE is turned off in the defconfig. If
> I set CONFIG_HIGHMEM_64G which turns on CONFIG_X86_PAE the problem
> goes away. I can't tell if this is an indication of magically hiding
> the TLB problem or if there is an issue with !X86_PAE invalidation.

I found my difference. I normally run qemu with "-cpu host" which
makes the failure go away. With "-cpu kvm64", I see the rodata_test
failure immediately. Seems like this may be a kvm cpu feature
emulation bug? I'll see if I can find the specific cpu feature in the
morning...

-Kees

-- 
Kees Cook
Pixel Security