[PATCH v3 net] ipv6: fib6: fix NULL deref in fib6_walk_continue() on multi-batch dump

[PATCH v3 net] ipv6: fib6: fix NULL deref in fib6_walk_continue() on multi-batch dump

[Pengfei Zhang]

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 <zhangfeionline@gmail.com>
---
v3:
 - Fix Author/SOB email mismatch (use gmail for both)
 - Drop "RTNL lock is released" from commit message (RTNL removed from IPv6 FIB)
 - Reorder local variables to follow reverse xmas tree
 - Move blank line after continue for readability

v2:
 - Add Fixes tag

v2: https://lore.kernel.org/netdev/20260625044101.939070-1-zhangfeionline@gmail.com/
v1: https://lore.kernel.org/netdev/20260624171156.822055-1-zhangfeionline@gmail.com/

 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 fc95738de..a130cdfae 100644
--- a/net/ipv6/ip6_fib.c
+++ b/net/ipv6/ip6_fib.c
@@ -636,12 +636,12 @@ 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 e = 0, s_e;
 	struct hlist_head *head;
 	struct fib6_walker *w;
 	struct fib6_table *tb;
 	unsigned int h, s_h;
 	int err = 0;
+	u32 s_id;
 
 	rcu_read_lock();
 	if (cb->strict_check) {
@@ -701,23 +701,22 @@ 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];
 
-	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;
 			err = fib6_dump_table(tb, skb, cb);
 			if (err != 0)
 				goto out;
-next:
-			e++;
 		}
 	}
 out:
-	cb->args[1] = e;
 	cb->args[0] = h;
 
 unlock:
-- 
2.34.1