Re: [PATCH v8 3/8] Use atomic_t for ucounts reference counting

[Kees Cook <keescook@chromium.org>]

On Wed, Mar 10, 2021 at 01:01:28PM +0100, Alexey Gladkov wrote:
> The current implementation of the ucounts reference counter requires the
> use of spin_lock. We're going to use get_ucounts() in more performance
> critical areas like a handling of RLIMIT_SIGPENDING.

This really looks like it should be refcount_t. I read the earlier
thread[1] on this, and it's not clear to me that this is a "normal"
condition. I think there was a bug in that version (This appeared
to *instantly* crash at boot with mnt_init() calling alloc_mnt_ns()
calling inc_ucount()). The current code looks like just a "regular"
reference counter of the allocated struct ucounts. Overflow should be
very unexpected, yes? And operating on a "0" ucounts should be a bug
too, right?

> [...]
> +/* 127: arbitrary random number, small enough to assemble well */
> +#define refcount_zero_or_close_to_overflow(ucounts) \
> +	((unsigned int) atomic_read(&ucounts->count) + 127u <= 127u)

Regardless, this should absolutely not have "refcount" as a prefix. I
realize it's only used here, but that's needlessly confusing with regard
to it being atomic_t not refcount_t.

> +struct ucounts *get_ucounts(struct ucounts *ucounts)
> +{
> +	if (ucounts) {
> +		if (refcount_zero_or_close_to_overflow(ucounts)) {
> +			WARN_ONCE(1, "ucounts: counter has reached its maximum value");
> +			return NULL;
> +		}
> +		atomic_inc(&ucounts->count);
> +	}
> +	return ucounts;
> +}

I feel like this should just be:

	refcount_inc_not_zero(&ucounts->count);

Or, to address Linus's comment in the v3 series, change get_ucounts to
not return NULL first -- I can't see why that can ever happen in v8.

-Kees

[1] https://lore.kernel.org/lkml/116c7669744404364651e3b380db2d82bb23f983.1610722473.git.gladkov.alexey@gmail.com/

-- 
Kees Cook
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