[PATCH net v2 1/1] net/sched: cls_route: fix fastmap use-after-free and preserve netns teardown ordering

[Jamal Hadi Salim <jhs@mojatatu.com>]

The route4 classifier maintains a 16-slot fastmap cache that stores raw
struct route4_filter pointers indexed by (id, iif). The reader
(route4_classify) populates this cache via route4_set_fastmap() for every
classified packet that hits a filter. The writer (route4_delete,
route4_change) clears the cache via route4_reset_fastmap() before
RCU-deferred kfree of the filter.

This creates a UAF race:
 1. Reader walks the RCU-protected bucket chain, finds filter f
 2. Writer unlinks f, calls route4_reset_fastmap(), then tcf_queue_work()
 3. Reader calls route4_set_fastmap() and writes f into the cache
    *after* the writer's reset, caching a pointer about to be freed
 4. After the RCU grace period, kfree(f) executes
 5. Next classified packet on the same (id, iif) tuple hits the stale
    fastmap entry and reads f->res from freed memory

Reproduced with an mdelay(100) accelerator in route4_set_fastmap() and a
concurrent add/delete stress test (provided by both zdi and Santosh).
Both triggered KASAN slab-use-after-free reports in the route4 fastmap
paths.

Fix:
Move the fastmap flush to the writer side under RTNL, ordered after
synchronize_rcu(). This ensures stale fastmap entries written by
in-flight readers are visible (readers have finished) and no new readers
can find the unlinked filter, so the subsequent route4_reset_fastmap(head)
flushes them safely while head is valid under RTNL.

In route4_destroy() the synchronous __route4_delete_filter() fallback is
retained for the case where tcf_exts_get_net() returns false (netns is
being torn down in cleanup_net()). The trailing synchronize_rcu() before
kfree_rcu(head) drains any in-flight readers first, so the synchronous
free is safe. Removing the fallback and unconditionally deferring via
route4_queue_work() would race with tc_action_net_exit(): cleanup_net()
does not flush the tc_filter_wq workqueue, so the deferred work could run
after the action idrinfo has been freed, causing a use-after-free on
idrinfo->lock in __tcf_action_put().

Note: This fix will slow down deletes and filter add/replace, since
synchronize_rcu() is called under RTNL in route4_delete() and
route4_change(). An alternative fix would have been to introduce refcnt
to the head struct but that is a much bigger patch. We justify this as
a reasonable fix.

Fixes: 1109c00547fc ("net: sched: RCU cls_route")
Reported-by: zdi-disclosures@trendmicro.com
Reported-by: Santosh Kalluri <santosh.kalluri129@gmail.com>
Suggested-by: Santosh Kalluri <santosh.kalluri129@gmail.com>
Tested-by: Victor Nogueira <victor@mojatatu.com>
Tested-by: Santosh Kalluri <santosh.kalluri129@gmail.com>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
---
V1->V2
1. Fix based on feedback from the Sashikos
https://sashiko.dev/#/patchset/20260723105210.817079-1-jhs%40mojatatu.com
https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260723105210.817079-1-jhs%40mojatatu.com

Restore the "if (tcf_exts_get_net(&f->exts)) route4_queue_work(f);
else __route4_delete_filter(f);"
preserving the tcf_exts_get_net() contract for netns teardown.
---
 net/sched/cls_route.c | 34 +++++++++++++++++++++++++++++++---
 1 file changed, 31 insertions(+), 3 deletions(-)

diff --git a/net/sched/cls_route.c b/net/sched/cls_route.c
index bd6f945bd388..c7d0164c1e9f 100644
--- a/net/sched/cls_route.c
+++ b/net/sched/cls_route.c
@@ -307,6 +307,19 @@ static void route4_destroy(struct tcf_proto *tp, bool rtnl_held,
 			kfree_rcu(b, rcu);
 		}
 	}
+
+	/* All filters are unlinked; no new reader can find them on the
+	 * chain.  Wait for in-flight readers that may still hold a filter
+	 * pointer and have published it into the fastmap after we unlinked.
+	 * Then flush the stale entries while head is still valid under
+	 * RTNL, matching the route4_delete() pattern.  This also protects
+	 * the synchronous __route4_delete_filter() path above: when the
+	 * netns is dying, tcf_exts_get_net() returns false and the filter
+	 * is freed directly, but only after all readers have drained.
+	 */
+	synchronize_rcu();
+	route4_reset_fastmap(head);
+
 	kfree_rcu(head, rcu);
 }
 
@@ -334,10 +347,16 @@ static int route4_delete(struct tcf_proto *tp, void *arg, bool *last,
 			/* unlink it */
 			RCU_INIT_POINTER(*fp, rtnl_dereference(f->next));
 
-			/* Remove any fastmap lookups that might ref filter
-			 * notice we unlink'd the filter so we can't get it
-			 * back in the fastmap.
+			/* This code path assumes we have the RTNL lock.
+			 * We wait for in-flight readers that may still hold
+			 * the filter pointer and publish it into the fastmap
+			 * after we unlinked it.
+			 * Once done, no new reader can find the filter on the
+			 * chain, so the only stale fastmap entries are the
+			 * ones those readers just wrote. Flush them now
+			 * while head is still valid.
 			 */
+			synchronize_rcu();
 			route4_reset_fastmap(head);
 
 			/* Delete it */
@@ -558,6 +577,15 @@ static int route4_change(struct net *net, struct sk_buff *in_skb,
 		}
 	}
 
+	/* Assuming RTNL lock. Wait for in-flight readers that may still
+	 * hold a filter pointer and publish it into the fastmap after we
+	 * inserted the new filter (or unlinked fold when replacing).
+	 * Once they are done, flush the stale entries while head is still
+	 * valid.  Needed on both the creation and replace paths: on
+	 * creation, readers may have cached a ROUTE4_FAILURE entry for the
+	 * (id, iif) tuple that the new filter now matches.
+	 */
+	synchronize_rcu();
 	route4_reset_fastmap(head);
 	*arg = f;
 	if (fold) {
-- 
2.34.1