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

From: Daniel Borkmann
Date: Thu Mar 09 2017 - 08:05:22 EST

On 03/09/2017 06:36 AM, Kees Cook wrote:
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?


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

Interesting! Changing to "-cpu host" makes rodata_test succeed plus
my test_setmem and the test_bpf suite runs fine as well. Haven't seen
a corruption since. Switching back to "-cpu kvm64" I immediately see
mentioned issues again.

With regard to CPA_FLUSHTLB that Linus mentioned, when I investigated
code paths in change_page_attr_set_clr(), I did see that CPA_FLUSHTLB
was set each time we switched attrs and a cpa_flush_range() was
performed (with the correct number of pages and cache set to 0). That
would be a __flush_tlb_all() eventually.

Hmm, it indeed might seem likely that this could be an emulation bug.