RE: [PATCH 2/4] net/colo: Fix a "double free" crash to clear the conn_list

[Zhang, Chen]

> -----Original Message-----
> From: lizhijian@fujitsu.com <lizhijian@fujitsu.com>
> Sent: Monday, March 21, 2022 11:06 AM
> To: Zhang, Chen <chen.zhang@intel.com>; Jason Wang
> <jasowang@redhat.com>; lizhijian@fujitsu.com
> Cc: qemu-dev <qemu-devel@nongnu.org>; Like Xu <like.xu@linux.intel.com>
> Subject: Re: [PATCH 2/4] net/colo: Fix a "double free" crash to clear the
> conn_list
> 
> 
> 
> On 09/03/2022 16:38, Zhang Chen wrote:
> > We notice the QEMU may crash when the guest has too many incoming
> > network connections with the following log:
> >
> > 15197@1593578622.668573:colo_proxy_main : colo proxy connection
> > hashtable full, clear it
> > free(): invalid pointer
> > [1]    15195 abort (core dumped)  qemu-system-x86_64 ....
> >
> > This is because we create the s->connection_track_table with
> > g_hash_table_new_full() which is defined as:
> >
> > GHashTable * g_hash_table_new_full (GHashFunc hash_func,
> >                         GEqualFunc key_equal_func,
> >                         GDestroyNotify key_destroy_func,
> >                         GDestroyNotify value_destroy_func);
> >
> > The fourth parameter connection_destroy() will be called to free the
> > memory allocated for all 'Connection' values in the hashtable when we
> > call g_hash_table_remove_all() in the connection_hashtable_reset().
> >
> > It's unnecessary because we clear the conn_list explicitly later, and
> > it's buggy when other agents try to call connection_get() with the
> > same connection_track_table.
> >
> > Signed-off-by: Like Xu <like.xu@linux.intel.com>
> > Signed-off-by: Zhang Chen <chen.zhang@intel.com>
> > ---
> >   net/colo-compare.c    | 2 +-
> >   net/filter-rewriter.c | 2 +-
> >   2 files changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/colo-compare.c b/net/colo-compare.c index
> > 62554b5b3c..ab054cfd21 100644
> > --- a/net/colo-compare.c
> > +++ b/net/colo-compare.c
> > @@ -1324,7 +1324,7 @@ static void
> colo_compare_complete(UserCreatable *uc, Error **errp)
> >       s->connection_track_table =
> g_hash_table_new_full(connection_key_hash,
> >                                                         
> > connection_key_equal,
> >                                                         g_free,
> > -                                                      connection_destroy);
> > +                                                      NULL);
> 
> 
> 202 /* if not found, create a new connection and add to hash table */
> 203 Connection *connection_get(GHashTable *connection_track_table,
> 204                            ConnectionKey *key,
> 205                            GQueue *conn_list)
> 206 {
> 207     Connection *conn = g_hash_table_lookup(connection_track_table,
> key);
> 208
> 209     if (conn == NULL) {
> 210         ConnectionKey *new_key = g_memdup(key, sizeof(*key));
> 211
> 212         conn = connection_new(key);
> 213
> 214         if (g_hash_table_size(connection_track_table) >
> HASHTABLE_MAX_SIZE) {
> 215             trace_colo_proxy_main("colo proxy connection hashtable full,"
> 216                                   " clear it");
> 217 connection_hashtable_reset(connection_track_table);
> 
> 197 void connection_hashtable_reset(GHashTable *connection_track_table)
> 198 {
> 199 g_hash_table_remove_all(connection_track_table);
> 200 }
> 
> IIUC,  above subroutine will do some cleanup explicitly. And before your
> patch, connection_hashtable_reset() will release all keys and their values in
> this hashtable. But now, you remove all keys and just one value(conn_list)
> instead. Does it means other values will be leaked ?

Thanks Zhijian. Re-think about it. Yes, I think you are right.
It looks need a lock to avoid into connection_get() when triggered the clear 
hashtable operation.
What do you think?

Thanks
Chen


> 
> 
> 218 /*
> 219              * clear the conn_list
> 220 */
> 221             while (!g_queue_is_empty(conn_list)) {
> 222 connection_destroy(g_queue_pop_head(conn_list));
> 223 }
> 224 }
> 225
> 226         g_hash_table_insert(connection_track_table, new_key, conn);
> 227 }
> 228
> 229     return conn;
> 230 }
> 
> 
> Thanks
> Zhijian
> 
> >
> >       colo_compare_iothread(s);
> >
> > diff --git a/net/filter-rewriter.c b/net/filter-rewriter.c index
> > bf05023dc3..c18c4c2019 100644
> > --- a/net/filter-rewriter.c
> > +++ b/net/filter-rewriter.c
> > @@ -383,7 +383,7 @@ static void colo_rewriter_setup(NetFilterState *nf,
> Error **errp)
> >       s->connection_track_table =
> g_hash_table_new_full(connection_key_hash,
> >                                                         
> > connection_key_equal,
> >                                                         g_free,
> > -                                                      connection_destroy);
> > +                                                      NULL);
> >       s->incoming_queue =
> qemu_new_net_queue(qemu_netfilter_pass_to_next, nf);
> >   }
> >