Re: [PATCH v3 6/7] x86/alternatives: use temporary mm for text poking

[Nadav Amit <namit@vmware.com>]

From: Peter Zijlstra
Sent: November 6, 2018 at 1:11:19 PM GMT
> To: Nadav Amit <namit@vmware.com>
> Cc: Ingo Molnar <mingo@redhat.com>, LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>, H. Peter Anvin <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>, Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>, Andy Lutomirski <luto@kernel.org>, Kees Cook <keescook@chromium.org>, Dave Hansen <dave.hansen@intel.com>, Masami Hiramatsu <mhiramat@kernel.org>
> Subject: Re: [PATCH v3 6/7] x86/alternatives: use temporary mm for text poking
> 
> 
> On Tue, Nov 06, 2018 at 09:20:19AM +0100, Peter Zijlstra wrote:
> 
>> By our current way of thinking, kmap_atomic simply is not correct.
> 
> Something like the below; which weirdly builds an x86_32 kernel.
> Although I imagine a very sad one.
> 
> ---
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index ba7e3464ee92..e273f3879d04 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1449,6 +1449,16 @@ config PAGE_OFFSET
> config HIGHMEM
> 	def_bool y
> 	depends on X86_32 && (HIGHMEM64G || HIGHMEM4G)
> +	depends on !SMP || BROKEN
> +	help
> +	  By current thinking kmap_atomic() is broken, since it relies on per
> +	  CPU PTEs in the global (kernel) address space and relies on CPU local
> +	  TLB invalidates to completely invalidate these PTEs. However there is
> +	  nothing that guarantees other CPUs will not speculatively touch upon
> +	  'our' fixmap PTEs and load then into their TLBs, after which our
> +	  local TLB invalidate will not invalidate them.
> +
> +	  There are AMD chips that will #MC on inconsistent TLB states.
> 
> config X86_PAE
> 	bool "PAE (Physical Address Extension) Support”

Please help me understand the scenario you are worried about. I see several
(potentially) concerning situations due to long lived mappings:

1. Inconsistent cachability in the PAT (between two different mappings of
the same physical memory), causing memory ordering issues.

2. Inconsistent access-control (between two different mappings of the same
physical memory), allowing to circumvent security hardening mechanisms.

3. Invalid cachability in the PAT for MMIO, causing #MC

4. Faulty memory being mapped, causing #MC

5. Some potential data leakage due to long lived mappings

The #MC you mention, I think, regards something that resembles (3) -
speculative page-walks using cachable memory caused #MC when this memory was
set on MMIO region. This memory, IIUC, was mistakenly presumed to be used by
page-tables, so I don’t see how it is relevant for kmap_atomic().

As for the other situations, excluding (2), which this series is intended to
deal with, I don’t see a huge problem which cannot be resolved in different
means.