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

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Date:   Wed, 8 Mar 2017 21:36:01 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Laura Abbott <labbott@...hat.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Peter Anvin <hpa@...or.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Network Development <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, LKP <lkp@...org>,
        ast@...com, "the arch/x86 maintainers" <x86@...nel.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request
 at 0000a7cf

On Wed, Mar 8, 2017 at 3:55 PM, Laura Abbott <labbott@...hat.com> wrote:
> On 03/08/2017 02:36 PM, Kees Cook wrote:
>> On Wed, Mar 8, 2017 at 2:27 PM, Daniel Borkmann <daniel@...earbox.net> 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