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Date: Wed, 31 Jan 2018 09:03:10 +0100
From: Ingo Molnar <>
To: "Van De Ven, Arjan" <>
Cc: "Williams, Dan J" <>,
	Linus Torvalds <>,
	Thomas Gleixner <>,
	linux-arch <>,
	Cyril Novikov <>,
	Kernel Hardening <>,
	Peter Zijlstra <>,
	Catalin Marinas <>, X86 ML <>,
	Will Deacon <>,
	Russell King <>,
	Ingo Molnar <>,
	Greg KH <>,
	"H. Peter Anvin" <>, Alan Cox <>,
	Linux Kernel Mailing List <>,
	Peter Zijlstra <>
Subject: Re: [PATCH v5 02/12] array_idx: sanitize speculative array

* Van De Ven, Arjan <> wrote:

> > > IOW, is there some work on tooling/analysis/similar? Not asking for
> > > near-term, but more of a "big picture" question..
> short term there was some extremely rudimentary static analysis done. clearly 
> that has extreme limitations both in insane rate of false positives, and missing 
> cases.

What was the output roughly, how many suspect places that need array_idx_nospec() 
handling: a few, a few dozen, a few hundred or a few thousand?

I'm guessing 'static tool emitted hundreds of sites with many false positives 
included, but the real sites are probably a few dozen' - but that's really just a 
very, very wild guess.

Our whole mindset and approach with these generic APIs obviously very much depends 
on the order of magnitude:

- For array users up to a few dozen per 20 MLOC code base accumulated over 20
  years, and with no more than ~1 new site per kernel release we can probably
  do what we do for buffer overflows: static analyze and fix via 

- If it's more than a few dozen then intuitively I'd also be very much in favor of
  compiler help: for example trickle down __user annotations that Sparse uses some
  more and let the compiler sanitize indices or warn about them - without hurting 
  performance of in-kernel array handling.

- If it's "hundreds" then probably both the correctness and the maintenance
  aspects won't scale well to a 20+ MLOC kernel code base - in that case we _have_
  to catch the out of range values at a much earlier stage, at the system call and
  other system ABI level, and probably need to do it via a self-maintaining 
  approach such as annotations/types that also warns about 'unsanitized' uses. But 
  filtering earlier has its own problems as well: mostly the layering violations 
  (high level interfaces often don't know the safe array index range) can create 
  worse bugs and more fragility than what is being fixed ...

- If it's "thousands" then it _clearly_ won't scale and we probably need compiler 
  help: i.e. a compiler that tracks ranges and automatically sanitizes indices. 
  This will have some performance effect, but should result in almost immediate 

Also, IMHO any tooling should very much be open source: this isn't the kind of bug 
that can be identified via code review, so there's no fall-back detection method 
like we have for buffer overflows.

Anyway, the approach taken very much depends on the order of magnitude of such 
affected array users we are targeting ...



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