[PATCH 6.6 086/261] ipv6: fib6: fix NULL deref in fib6_walk_continue() on multi-batch dump

6.6-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Pengfei Zhang <[email protected]>

commit 9facb861dc6b9b9ea9793ef5032a9a826f7a4229 upstream.

inet6_dump_fib() saves its progress in cb->args[1] as a positional
index within the current hash chain.  Between batches, a concurrent
fib6_new_table() can insert a new table at the chain head, shifting
all existing entries.  The saved index then lands on a different
table, causing fib6_dump_table() to set w->root to the wrong table
while w->node still points into the previous one.
fib6_walk_continue() dereferences w->node->parent (NULL) and panics:

  BUG: kernel NULL pointer dereference, address: 0000000000000008
  RIP: 0010:fib6_walk_continue+0x6e/0x170
  Call Trace:
   <TASK>
   fib6_dump_table.isra.0+0xc5/0x240
   inet6_dump_fib+0xf6/0x420
   rtnl_dumpit+0x30/0xa0
   netlink_dump+0x15b/0x460
   netlink_recvmsg+0x1d6/0x2a0
   ____sys_recvmsg+0x17a/0x190

Fix by storing tb->tb6_id in cb->args[1] instead of a positional
index.  On resume, skip entries until the id matches; a concurrent
head-insert can never match the saved id, so the walker always
resumes on the correct table.

Fixes: 1b43af5480c3 ("[IPV6]: Increase number of possible routing tables to 2^32")
Signed-off-by: Pengfei Zhang <[email protected]>
Reviewed-by: Ido Schimmel <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>
[Adapted to 5.10/6.1/6.6: inet6_dump_fib() there predates 22e36ea9f5d7
 and 5fc68320c1fb, so the return variable is "res" not "err" and the
 RCU-protected hash walk exits via "out_unlock" instead of "unlock".
 Context-only change; the fix itself is identical.]
Signed-off-by: Pengfei Zhang <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>
---
 net/ipv6/ip6_fib.c | 17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
index 7a28807ca4464..383b07269a499 100644
--- a/net/ipv6/ip6_fib.c
+++ b/net/ipv6/ip6_fib.c
@@ -627,11 +627,11 @@ static int inet6_dump_fib(struct sk_buff *skb, struct netlink_callback *cb)
 	const struct nlmsghdr *nlh = cb->nlh;
 	struct net *net = sock_net(skb->sk);
 	unsigned int h, s_h;
-	unsigned int e = 0, s_e;
 	struct fib6_walker *w;
 	struct fib6_table *tb;
 	struct hlist_head *head;
 	int res = 0;
+	u32 s_id;
 
 	if (cb->strict_check) {
 		int err;
@@ -689,25 +689,24 @@ static int inet6_dump_fib(struct sk_buff *skb, struct netlink_callback *cb)
 	}
 
 	s_h = cb->args[0];
-	s_e = cb->args[1];
+	s_id = cb->args[1];
 
 	rcu_read_lock();
-	for (h = s_h; h < FIB6_TABLE_HASHSZ; h++, s_e = 0) {
-		e = 0;
+	for (h = s_h; h < FIB6_TABLE_HASHSZ; h++, s_id = 0) {
 		head = &net->ipv6.fib_table_hash[h];
 		hlist_for_each_entry_rcu(tb, head, tb6_hlist) {
-			if (e < s_e)
-				goto next;
+			if (s_id && tb->tb6_id != s_id)
+				continue;
+
+			s_id = 0;
+			cb->args[1] = tb->tb6_id;
 			res = fib6_dump_table(tb, skb, cb);
 			if (res != 0)
 				goto out_unlock;
-next:
-			e++;
 		}
 	}
 out_unlock:
 	rcu_read_unlock();
-	cb->args[1] = e;
 	cb->args[0] = h;
 out:
 	res = res < 0 ? res : skb->len;
-- 
2.53.0