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

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Message-Id: <20170309.100830.994695882061843653.davem@davemloft.net>
Date:   Thu, 09 Mar 2017 10:08:30 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     daniel@...earbox.net
Cc:     tglx@...utronix.de, keescook@...omium.org, labbott@...hat.com,
        torvalds@...ux-foundation.org, mingo@...nel.org, hpa@...or.com,
        fengguang.wu@...el.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, lkp@...org, ast@...com,
        x86@...nel.org
Subject: Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging
 request at 0000a7cf

From: Daniel Borkmann <daniel@...earbox.net>
Date: Thu, 09 Mar 2017 18:51:03 +0100

> I added some debugging around __native_flush_tlb_global_irq_disabled()
> and if I understand it correctly, the idea of cr4 is that we need to
> toggle X86_CR4_PGE in order to trigger a TLB flush.
> 
> What I see is that original cr4 is 0x610. The cpu_tlbstate.cr4 is
> consistent to native_read_cr4() and since cr4 is != 0, it tells me
> based on the comment in native_read_cr4() that cr4 seems to be
> supported. Thus, meaning we end up with writing ...
> 
>   native_write_cr4(0x610);
>   native_write_cr4(0x610);
> 
> ... twice, and this just doesn't trigger the desired TLB flush. I
> changed the code into the following ...
> 
>         cr4 = this_cpu_read(cpu_tlbstate.cr4);
>         /* clear PGE */
> -       native_write_cr4(cr4 & ~X86_CR4_PGE);
> +       native_write_cr4(cr4 ^ X86_CR4_PGE);
>         /* write old PGE again and flush TLBs */
>         native_write_cr4(cr4);
> 
> ... and the test cases seem to be working for me now with "-cpu
> kvm64",
> so that seems to trigger the TLB we were missing.

Great detective work Daniel.