[cfe-dev] making -ftrivial-auto-var-init=zero a first-class option

Kees Cook via cfe-dev cfe-dev at lists.llvm.org
Tue Apr 21 16:19:38 PDT 2020

On Tue, Apr 21, 2020 at 06:29:07PM -0400, Hubert Tong wrote:
> On Tue, Apr 21, 2020 at 5:20 PM Kees Cook via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> > https://clangbuiltlinux.github.io/CBL-meetup-2020-slides/glider/Fighting_uninitialized_memory_%40_CBL_Meetup_2020.pdf
> > tl;dr: zero-init tended to be half the cost of pattern-init, though it
> > varied based on workload, and binary size impact fell over 95% going
> > from pattern-init to zero-init.
> >
> This does not seem to indicate why zero-init is preferred over a default of
> using no explicit policy in production.

Do you mean "leaving things uninitialized" when you say "no explicit
policy"? Maybe I've misunderstood. Google's goal of using auto-init is
to eliminate uninitialized variables in production as a security
defense. When examining zero-init vs pattern-init, there is a clear
advantage on performance and size for zero-init.

> > as good. There are certainly exceptions here, but the bulk of the
> > historical record on how "uninitialized" variables have been used in
> >
> Maybe an explanation of the scare quotes around "uninitialized" would help
> clarify your position.

Ah, sorry, I always use quotes (they are not intended to scare but to
clarify) when discussing uninitialized variables in real-world contexts,
because they are, of course, not uninitialized in the sense of them
not having a value. The RAM contents have a value. Many people without
compiler backgrounds think of such variables as being uncontrollable
or meaningless, when in fact they are usually highly controllable by an
attacker, etc.

> > Google (Android, Chrome OS)
> >
> > Both Android and Chrome OS initially started using pattern-init, but due
> > to each of: the performance characteristics, the binary size changes, and
> > the less robust security stance, both projects have recently committed
> > to switching to zero-init.
> >
> I'm not sure that this is clear in terms of whether the statements apply to
> debug/development or production. I don't think pattern-init is meant to be
> a tool for production builds, which leads me to think that the above
> statement is about debug builds, at which point I'm thinking that using
> zero-init only serves to hide problems.

The context for Google's use of zero-init was meant here to be about
production builds.

> > Upstream Linux kernel
> >
> > Linus Torvalds has directly stated that he wants zero-init:
> > "So I'd like the zeroing of local variables to be a native compiler
> > option..."
> > "This, btw, is why I also think that the "initialize with poison" is
> > pointless and wrong."
> >
> > https://lore.kernel.org/lkml/CAHk-=wgTM+cN7zyUZacGQDv3DuuoA4LORNPWgb1Y_Z1p4iedNQ@mail.gmail.com/
> > Unsurprisingly, I strongly agree. ;)
> >
> I don't see why claiming that pattern-init is bad helps make the case for
> zero-init.

Perhaps I did not express it well enough, but both have meaningful and
important uses. My goal here is to illustrate how zero-init is being
used (or preferred) in many real situations, as an argument for why it
should not be hidden behind what some might see as a scary enable flag.

> > Apple
> >
> > I can't speak meaningfully here, but I've heard rumors that they are
> > depending on zero-init as well. Perhaps someone there can clarify how
> > they are using these features?
> >
> There's a difference between "depending on zero-init" (as in, the group in
> question is okay with relying on implicit zeroing on code reviews, etc.)
> and the use of zero-init as some sort of defence-in-depth approach. Are
> these rumours clear as to which?

My understanding was the latter, but I hope to find out for real via
this thread! :) It's not clear to me either.

> > So, while I understand the earlier objections to zero-init from a
> > "language fork" concern, I think this isn't a position that can really
> > stand up to the reality of how many projects are using the feature (even
> > via non-Clang compilers). Given that so much code is going to be built
> > using zero-init, what's the best way for Clang to adapt here?
> It happens that there is zero-init and it's at least close enough to what
> these projects want, but it is actually what they need?

Yes, it's expressly what is desired from a security perspective. (And
quite to the relief of that same community, comes with the least
performance impact, which is an unfortunately uncommon scenario in
security flaw mitigations.) I tried to detail that earlier in my email
where it's directly what is indicated as a meaningful defense against
the long history of real-world "uninitialized" variable attacks: setting
everything to zero is the best defense for the entire class of flaws.

Kees Cook

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