Patch "ipv4: Fix use-after-free when flushing FIB tables" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

ipv4: Fix use-after-free when flushing FIB tables

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
ipv4-fix-use-after-free-when-flushing-fib-tables.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Ido Schimmel <[email protected]>
Date: Wed, 20 Dec 2017 19:34:19 +0200
Subject: ipv4: Fix use-after-free when flushing FIB tables

From: Ido Schimmel <[email protected]>

[ Upstream commit b4681c2829e24943aadd1a7bb3a30d41d0a20050 ]

Since commit 0ddcf43d5d4a ("ipv4: FIB Local/MAIN table collapse") the local table uses the same trie allocated for the main table when custom rules are not in use.

When a net namespace is dismantled, the main table is flushed and freed (via an RCU callback) before the local table. In case the callback is invoked before the local table is iterated, a use-after-free can occur.

Fix this by iterating over the FIB tables in reverse order, so that the main table is always freed after the local table.

v3: Reworded comment according to Alex's suggestion.
v2: Add a comment to make the fix more explicit per Dave's and Alex's feedback.

---
 net/ipv4/fib_frontend.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

@@ -1253,14 +1253,19 @@ fail:
 
 static void ip_fib_net_exit(struct net *net)
 {
-	unsigned int i;
+	int i;
 
 	rtnl_lock();
 #ifdef CONFIG_IP_MULTIPLE_TABLES
 	RCU_INIT_POINTER(net->ipv4.fib_main, NULL);
 	RCU_INIT_POINTER(net->ipv4.fib_default, NULL);
 #endif
-	for (i = 0; i < FIB_TABLE_HASHSZ; i++) {
+	/* Destroy the tables in reverse order to guarantee that the
+	 * local table, ID 255, is destroyed before the main table, ID
+	 * 254. This is necessary as the local table may contain
+	 * references to data contained in the main table.
+	 */
+	for (i = FIB_TABLE_HASHSZ - 1; i >= 0; i--) {
 		struct hlist_head *head = &net->ipv4.fib_table_hash[i];
 		struct hlist_node *tmp;
 		struct fib_table *tb;

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/ipv4-fix-use-after-free-when-flushing-fib-tables.patch