From: Junio C Hamano <gitster@pobox.com>
Date: Wed, 14 Oct 2015 21:17:34 GMT
Subject: Re: [PATCH v2] merge: fix cache_entry use-after-free
Message-ID: <xmqqk2qp6tg1.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <1444687413-928-1-git-send-email-dturner@twitter.com>

David Turner <dturner@twopensource.com> writes:

> +	unsigned int ref_count; /* count the number of refs to this in dir_hash */

Me makes a mental note of the type used...

> @@ -213,6 +214,32 @@ struct cache_entry {
>  struct pathspec;
>  
>  /*
> + * Increment the cache_entry reference count.  Should be called
> + * whenever a pointer to a cache_entry is retained in a data structure,
> + * thus marking it as alive.
> + */
> +static inline void add_ce_ref(struct cache_entry *ce)
> +{
> +	assert(ce != NULL && ce->ref_count >= 0);

... and notices that ce->ref_count will always be non-negative here


> +	ce->ref_count++;
> +}
> +
> +/*
> + * Decrement the cache_entry reference count.  Should be called whenever
> + * a pointer to a cache_entry is dropped.  Once the counter drops to 0
> + * the cache_entry memory will be safely freed.
> + */
> +static inline void drop_ce_ref(struct cache_entry *ce)
> +{
> +	if (ce != NULL) {
> +		assert(ce->ref_count >= 0);

... and here.

By not checking integer overflow/wraparound, the code is assuming
that a ce entry will never referenced more than 4 billion times on
32-bit platform.  And that is a sensible assumption as there aren't
that many pointers in the address space to make that many reference
anyway.

Perhaps the code can assume the number won't be more than 2 billion
and use a signed type instead for the reference counting?