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

From	Thomas Gleixner <tglx@linutronix.de>
Newsgroups	linux.kernel
Subject	Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request at 0000a7cf
Date	2017-03-09 14:50 +0100
Message-ID	<tj6C6-5St-7@gated-at.bofh.it> (permalink)
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On Wed, 8 Mar 2017, Linus Torvalds wrote:

> Adding x86 people too, since this seems to be something off about
> ARCH_HAS_SET_MEMORY for x86-32.
> 
> The code seems to be shared between x86-32 and 64, I'm not seeing why
> set_memory_r[ow]() should fail on one but not the other.

Indeed.

> Considering that it seems to be flaky even on 32-bit, maybe it's
> timing-related, or possibly related to TLB sizes or whatever (ie more
> likely hidden by a larger TLB on more modern hardware?)

The only difference I can see is the way how __tlb_flush_all() is
happening. We have 3 variants:

   invpcid_flush_all()  - depends on X86_FEATURE_INVPCID and X86_FEATURE_PGE
   cr4 based flush	- depends on X86_FEATURE_PGE
   cr3 based flush

No idea which variant is used in that failure case.

> Anyway, just looking at change_page_attr_set_clr(), I notice that the
> page alias checking treats NX specially:
> 
>         /* No alias checking for _NX bit modifications */
>         checkalias = (pgprot_val(mask_set) | pgprot_val(mask_clr)) != _PAGE_NX;
> 
> which seems insane. Why would NX be different from other protection
> bits (like _PAGE_RW)?

The reason is that the alias mapping should never be executable at all.

Thanks,

	tglx

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Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Fengguang Wu <fengguang.wu@intel.com> - 2017-03-02 21:30 +0100
  Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Daniel Borkmann <daniel@iogearbox.net> - 2017-03-02 22:40 +0100
    Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Linus Torvalds <torvalds@linux-foundation.org> - 2017-03-08 20:30 +0100
      Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Kees Cook <keescook@chromium.org> - 2017-03-08 23:40 +0100
        Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Daniel Borkmann <daniel@iogearbox.net> - 2017-03-09 00:20 +0100
        Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Laura Abbott <labbott@redhat.com> - 2017-03-09 01:30 +0100
          Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Kees Cook <keescook@chromium.org> - 2017-03-09 06:40 +0100
            Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Daniel Borkmann <daniel@iogearbox.net> - 2017-03-09 14:10 +0100
              Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Thomas Gleixner <tglx@linutronix.de> - 2017-03-09 14:20 +0100
                Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Daniel Borkmann <daniel@iogearbox.net> - 2017-03-09 15:10 +0100
                Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Thomas Gleixner <tglx@linutronix.de> - 2017-03-09 16:00 +0100
                Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Daniel Borkmann <daniel@iogearbox.net> - 2017-03-09 19:00 +0100
                Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging  request at 0000a7cf David Miller <davem@davemloft.net> - 2017-03-09 19:10 +0100
                Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Linus Torvalds <torvalds@linux-foundation.org> - 2017-03-09 19:20 +0100
                Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Linus Torvalds <torvalds@linux-foundation.org> - 2017-03-09 19:20 +0100
                Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Daniel Borkmann <daniel@iogearbox.net> - 2017-03-09 19:40 +0100
                Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Daniel Borkmann <daniel@iogearbox.net> - 2017-03-09 22:40 +0100
                Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Borislav Petkov <bp@suse.de> - 2017-03-09 23:10 +0100
                Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Daniel Borkmann <daniel@iogearbox.net> - 2017-03-09 23:20 +0100
                Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Borislav Petkov <bp@suse.de> - 2017-03-09 23:50 +0100
                Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Linus Torvalds <torvalds@linux-foundation.org> - 2017-03-10 00:30 +0100
                Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Borislav Petkov <bp@suse.de> - 2017-03-10 00:50 +0100
                Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Daniel Borkmann <daniel@iogearbox.net> - 2017-03-10 01:20 +0100
                Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Borislav Petkov <bp@suse.de> - 2017-03-12 22:50 +0100
                Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Borislav Petkov <bp@suse.de> - 2017-03-09 23:20 +0100
                Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Daniel Borkmann <daniel@iogearbox.net> - 2017-03-09 16:00 +0100
                Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Linus Torvalds <torvalds@linux-foundation.org> - 2017-03-09 18:50 +0100
      Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Linus Torvalds <torvalds@linux-foundation.org> - 2017-03-08 23:50 +0100
        Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Fengguang Wu <fengguang.wu@intel.com> - 2017-03-09 02:40 +0100
      Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request  at 0000a7cf Thomas Gleixner <tglx@linutronix.de> - 2017-03-09 14:50 +0100

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