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

From: Daniel Borkmann <daniel@iogearbox.net>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Kees Cook <keescook@chromium.org>,
	Laura Abbott <labbott@redhat.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Ingo Molnar <mingo@kernel.org>, Peter Anvin <hpa@zytor.com>,
	Fengguang Wu <fengguang.wu@intel.com>,
	Network Development <netdev@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>, LKP <lkp@01.org>,
	ast@fb.com, the arch/x86 maintainers <x86@kernel.org>,
	"David S. Miller" <davem@davemloft.net>
Subject: Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request at 0000a7cf
Date: Thu, 09 Mar 2017 14:25:58 +0100	[thread overview]
Message-ID: <58C157E6.1010909@iogearbox.net> (raw)
In-Reply-To: <alpine.DEB.2.20.1703091409080.3521@nanos>

On 03/09/2017 02:10 PM, Thomas Gleixner wrote:
> On Thu, 9 Mar 2017, Daniel Borkmann wrote:
>> 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.
>
> Which variant of __flush_tlb_all() is used when the test fails?
>
> Check for the following flags in /proc/cpuinfo: pge invpcid

I added the following and booted with both variants:

printk("X86_FEATURE_PGE:%u\n",     static_cpu_has(X86_FEATURE_PGE));
printk("X86_FEATURE_INVPCID:%u\n", static_cpu_has(X86_FEATURE_INVPCID));

"-cpu host" gives:

[    8.326117] X86_FEATURE_PGE:1
[    8.326381] X86_FEATURE_INVPCID:1

"-cpu kvm64" gives:

[    8.517069] X86_FEATURE_PGE:1
[    8.517393] X86_FEATURE_INVPCID:0

Thanks,
Daniel