commit efa95b01da18 ("netpoll: fix use after free") incorrectly
ignored the refcount and prematurely set dev->npinfo to NULL during
netpoll cleanup, leading to improper behavior and memory leaks.
Scenario causing lack of proper cleanup:
1) A netpoll is associated with a NIC (e.g., eth0) and netdev->npinfo is
allocated, and refcnt = 1
- Keep in mind that npinfo is shared among all netpoll instances. In
this case, there is just one.
2) Another netpoll is also associated with the same NIC and
npinfo->refcnt += 1.
- Now dev->npinfo->refcnt = 2;
- There is just one npinfo associated to the netdev.
3) When the first netpolls goes to clean up:
- The first cleanup succeeds and clears np->dev->npinfo, ignoring
refcnt.
- It basically calls `RCU_INIT_POINTER(np->dev->npinfo, NULL);`
- Set dev->npinfo = NULL, without proper cleanup
- No ->ndo_netpoll_cleanup() is either called
4) Now the second target tries to clean up
- The second cleanup fails because np->dev->npinfo is already NULL.
* In this case, ops->ndo_netpoll_cleanup() was never called, and
the skb pool is not cleaned as well (for the second netpoll
instance)
- This leaks npinfo and skbpool skbs, which is clearly reported by
kmemleak.
Revert commit efa95b01da18 ("netpoll: fix use after free") and adds
clarifying comments emphasizing that npinfo cleanup should only happen
once the refcount reaches zero, ensuring stable and correct netpoll
behavior.
Cc: <sta...@vger.kernel.org> # 3.17.x
Cc: Jay Vosburgh <j...@jvosburgh.net>
Fixes: efa95b01da18 ("netpoll: fix use after free")
Signed-off-by: Breno Leitao <lei...@debian.org>
Reviewed-by: Simon Horman <ho...@kernel.org>
---
net/core/netpoll.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)
diff --git a/net/core/netpoll.c b/net/core/netpoll.c
index 5f65b62346d4e..19676cd379640 100644
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -815,6 +815,10 @@ static void __netpoll_cleanup(struct netpoll *np)
if (!npinfo)
return;
+ /* At this point, there is a single npinfo instance per netdevice, and
+ * its refcnt tracks how many netpoll structures are linked to it. We
+ * only perform npinfo cleanup when the refcnt decrements to zero.
+ */
if (refcount_dec_and_test(&npinfo->refcnt)) {
const struct net_device_ops *ops;
@@ -824,8 +828,7 @@ static void __netpoll_cleanup(struct netpoll *np)
RCU_INIT_POINTER(np->dev->npinfo, NULL);
call_rcu(&npinfo->rcu, rcu_cleanup_netpoll_info);
- } else
- RCU_INIT_POINTER(np->dev->npinfo, NULL);
+ }
skb_pool_flush(np);
}
--
2.47.3
