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

From: David Turner <[email-masked]>
Date: Thu, 15 Oct 2015 15:02:44 -0400

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);
> > +		}
> > +	}
> > +}
>