From: "René Scharfe" <l.s.r@web.de>
To: David Turner <dturner@twopensource.com>
Cc: git@vger.kernel.org, Keith McGuigan <kmcguigan@twitter.com>
Subject: Re: [PATCH v3] merge: fix cache_entry use-after-free
Date: Thu, 15 Oct 2015 22:38:43 +0200 [thread overview]
Message-ID: <56200ED3.9000401@web.de> (raw)
In-Reply-To: <1444935764.20887.1.camel@twopensource.com>
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 <kmcguigan@twitter.com>
>>>
>>> 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 <kmcguigan@twitter.com>
>>> Signed-off-by: David Turner <dturner@twopensource.com>
>>> ---
>>>
>>> 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
next prev parent reply other threads:[~2015-10-15 20:38 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-10-14 22:07 [PATCH v3] merge: fix cache_entry use-after-free David Turner
2015-10-15 3:35 ` René Scharfe
2015-10-15 19:02 ` David Turner
2015-10-15 20:38 ` René Scharfe [this message]
2015-10-15 20:51 ` Junio C Hamano
2015-10-16 7:05 ` David Turner
2015-10-16 16:04 ` Junio C Hamano
2015-10-19 22:27 ` David Turner
2015-10-20 2:13 ` Junio C Hamano
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