Re: [PATCH v1 0/5] treewide cleanup of random integer usage

Kees Cook
2022-10-06T06:30:21Z

On Wed, Oct 05, 2022 at 11:48:39PM +0200, Jason A. Donenfeld wrote:
> Hi folks,
>
> This is a five part treewide cleanup of random integer handling. The
> rules for random integers are:
>
> - If you want a secure or an insecure random u64, use get_random_u64().
> - If you want a secure or an insecure random u32, use get_random_u32().
> * The old function prandom_u32() has been deprecated for a while now
> and is just a wrapper around get_random_u32().
> - If you want a secure or an insecure random u16, use get_random_u16().
> - If you want a secure or an insecure random u8, use get_random_u8().
> - If you want secure or insecure random bytes, use get_random_bytes().
> * The old function prandom_bytes() has been deprecated for a while now
> and has long been a wrapper around get_random_bytes().
> - If you want a non-uniform random u32, u16, or u8 bounded by a certain
> open interval maximum, use prandom_u32_max().
> * I say "non-uniform", because it doesn't do any rejection sampling or
> divisions. Hence, it stays within the prandom_* namespace.
>
> These rules ought to be applied uniformly, so that we can clean up the
> deprecated functions, and earn the benefits of using the modern
> functions. In particular, in addition to the boring substitutions, this
> patchset accomplishes a few nice effects:
>
> - By using prandom_u32_max() with an upper-bound that the compiler can
> prove at compile-time is ≤65536 or ≤256, internally get_random_u16()
> or get_random_u8() is used, which wastes fewer batched random bytes,
> and hence has higher throughput.
>
> - By using prandom_u32_max() instead of %, when the upper-bound is not a
> constant, division is still avoided, because prandom_u32_max() uses
> a faster multiplication-based trick instead.
>
> - By using get_random_u16() or get_random_u8() in cases where the return
> value is intended to indeed be a u16 or a u8, we waste fewer batched
> random bytes, and hence have higher throughput.
>
> So, based on those rules and benefits from following them, this patchset
> breaks down into the following five steps:
>
> 1) Replace `prandom_u32() % max` and variants thereof with
> prandom_u32_max(max).
>
> 2) Replace `(type)get_random_u32()` and variants thereof with
> get_random_u16() or get_random_u8(). I took the pains to actually
> look and see what every lvalue type was across the entire tree.
>
> 3) Replace remaining deprecated uses of prandom_u32() with
> get_random_u32().
>
> 4) Replace remaining deprecated uses of prandom_bytes() with
> get_random_bytes().
>
> 5) Remove the deprecated and now-unused prandom_u32() and
> prandom_bytes() inline wrapper functions.
>
> I was thinking of taking this through my random.git tree (on which this
> series is currently based) and submitting it near the end of the merge
> window, or waiting for the very end of the 6.1 cycle when there will be
> the fewest new patches brewing. If somebody with some treewide-cleanup
> experience might share some wisdom about what the best timing usually
> winds up being, I'm all ears.

It'd be nice to capture some (all?) of the above somewhere. Perhaps just
a massive comment in the header?

> I've CC'd get_maintainers.pl, which is a pretty big list. Probably some
> portion of those are going to bounce, too, and everytime you reply to
> this thread, you'll have to deal with a bunch of bounces coming
> immediately after. And a recipient list this big will probably dock my
> email domain's spam reputation, at least temporarily. Sigh. I think
> that's just how it goes with treewide cleanups though. Again, let me
> know if I'm doing it wrong.

I usually stick to just mailing lists and subsystem maintainers.

If any of the subsystems ask you to break this up (I hope not), I've got
this[1], which does a reasonable job of splitting a commit up into
separate commits for each matching subsystem.

Showing that a treewide change can be reproduced mechanically helps with
keeping it together as one bit treewide patch, too, I've found. :)

Thank you for the cleanup! The "u8 rnd = get_random_u32()" in the tree
has bothered me for a loooong time.

-Kees

--
Kees Cook