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

On Thu, 2015-10-15 at 05:35 +0200, René Scharfe wrote:

Am 15.10.2015 um 00:07 schrieb David Turner:

quoted

From: Keith McGuigan <redacted>

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 <redacted>
Signed-off-by: David Turner <redacted>
---

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.  

quoted

+		if (--ce->ref_count < 1) {
+			free(ce);
+		}
+	}
+}