When a timer is deleted and not re-armed in igmp_mod_timer(), or stopped
in igmp_stop_timer(), the code currently decrements the reference counter
of the multicast list entry @im using refcount_dec(&im->refcnt).

However, both functions can be called from the RCU reader path:
- igmp_mod_timer() via igmp_heard_query() -> for_each_pmc_rcu()
- igmp_stop_timer() via igmp_rcv() -> igmp_heard_report()

If the group im was concurrently removed from the list by ip_mc_dec_group(),
its reference count might have already been decremented to 1.

In this case, timer_delete() succeeds, and refcount_dec() decrements
the refcount from 1 to 0. Since refcount_dec() does not free the object
when it hits 0 (unlike ip_ma_put()), the im structure is leaked.

Fix this by using ip_ma_put(im) instead of refcount_dec(&im->refcnt),
and deferring the put until after the spinlock is released.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/ipv4/igmp.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/igmp.c b/net/ipv4/igmp.c
index f5f9763895641bf86bfcf9fd7fd7b06012fa4ece..957013e7f7a25f32d848822caab322147db3dc6e 100644
--- a/net/ipv4/igmp.c
+++ b/net/ipv4/igmp.c

@@ -217,13 +217,18 @@ static void ip_sf_list_clear_all(struct ip_sf_list *psf)
 
 static void igmp_stop_timer(struct ip_mc_list *im)
 {
+	bool put = false;
+
 	spin_lock_bh(&im->lock);
 	if (timer_delete(&im->timer))
-		refcount_dec(&im->refcnt);
+		put = true;
 	WRITE_ONCE(im->tm_running, 0);
 	WRITE_ONCE(im->reporter, 0);
 	im->unsolicit_count = 0;
 	spin_unlock_bh(&im->lock);
+
+	if (put)
+		ip_ma_put(im);
 }
 
 /* It must be called with locked im->lock */

@@ -266,6 +271,8 @@ static void igmp_ifc_start_timer(struct in_device *in_dev, int delay)
 
 static void igmp_mod_timer(struct ip_mc_list *im, int max_delay)
 {
+	bool put = false;
+
 	spin_lock_bh(&im->lock);
 	im->unsolicit_count = 0;
 	if (timer_delete(&im->timer)) {

@@ -275,10 +282,13 @@ static void igmp_mod_timer(struct ip_mc_list *im, int max_delay)
 			spin_unlock_bh(&im->lock);
 			return;
 		}
-		refcount_dec(&im->refcnt);
+		put = true;
 	}
 	igmp_start_timer(im, max_delay);
 	spin_unlock_bh(&im->lock);
+
+	if (put)
+		ip_ma_put(im);
 }
 
 

-- 
2.55.0.rc0.799.gd6f94ed593-goog