Subject: Re: [PATCH v3] merge: fix cache_entry use-after-free

From: René Scharfe <[email-masked]>
Date: Thu, 15 Oct 2015 22:38:43 +0200

Am 15.10.2015 um 21:02 schrieb David Turner:
> On Thu, 2015-10-15 at 05:35 +0200, René Scharfe wrote:
>> Am 15.10.2015 um 00:07 schrieb David Turner:
>>> From: Keith McGuigan <[email-masked]>
>>>
>>> During merges, we would previously free entries that we no longer need
>>> in the destination index.  But those entries might also be stored in
>>> the dir_entry cache, and when a later call to add_to_index found them,
>>> they would be used after being freed.
>>>
>>> To prevent this, add a ref count for struct cache_entry.  Whenever
>>> a cache entry is added to a data structure, the ref count is incremented;
>>> when it is removed from the data structure, it is decremented.  When
>>> it hits zero, the cache_entry is freed.
>>>
>>> Signed-off-by: Keith McGuigan <[email-masked]>
>>> Signed-off-by: David Turner <[email-masked]>
>>> ---
>>>
>>> Fix type of ref_count (from unsigned int to int).
>>>
>>>
>>>    cache.h        | 27 +++++++++++++++++++++++++++
>>>    name-hash.c    |  7 ++++++-
>>>    read-cache.c   |  6 +++++-
>>>    split-index.c  | 13 ++++++++-----
>>>    unpack-trees.c |  6 ++++--
>>>    5 files changed, 50 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/cache.h b/cache.h
>>> index 752031e..7906026 100644
>>> --- a/cache.h
>>> +++ b/cache.h
>>> @@ -149,6 +149,7 @@ struct stat_data {
>>>
>>>    struct cache_entry {
>>>    	struct hashmap_entry ent;
>>> +	int ref_count; /* count the number of refs to this in dir_hash */
>>>    	struct stat_data ce_stat_data;
>>>    	unsigned int ce_mode;
>>>    	unsigned int ce_flags;
>>> @@ -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);
>>> +	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);
>>
>> Shouldn't this be "> 0" to prevent double frees?
>
> No.  If the ref_count is 1, then there is still some reference to the
> ce.  If it is 0, there is no reference to it, and the next check (< 1)
> will succeed and the ce will get freed.
>
>>> +		if (--ce->ref_count < 1) {
>>> +			free(ce);
>>> +		}
>>> +	}
>>> +}

OK, let me think out loud, step by step:

Given ref_count == 1 then the assert passes, ref_count gets decremented 
to 0, which is less than 1, so ce is freed.

Given ref_count == 0 then the assert passes, refcount gets decremented 
to -1, which is less than 1, so ce is freed again.

Where did I go wrong?

René