Subject: [PATCH 1/6] ipv4: fix a RCU-list lock in fib_triestat_seq_show

From: Qian Cai <[email-masked]>
Date: Wed, 25 Mar 2020 18:01:00 -0400

[ Upstream commit fbe4e0c1b298b4665ee6915266c9d6c5b934ef4a ]

fib_triestat_seq_show() calls hlist_for_each_entry_rcu(tb, head,
tb_hlist) without rcu_read_lock() will trigger a warning,

 net/ipv4/fib_trie.c:2579 RCU-list traversed in non-reader section!!

 other info that might help us debug this:

 rcu_scheduler_active = 2, debug_locks = 1
 1 lock held by proc01/115277:
  #0: c0000014507acf00 (&p->lock){+.+.}-{3:3}, at: seq_read+0x58/0x670

 Call Trace:
  dump_stack+0xf4/0x164 (unreliable)
  lockdep_rcu_suspicious+0x140/0x164
  fib_triestat_seq_show+0x750/0x880
  seq_read+0x1a0/0x670
  proc_reg_read+0x10c/0x1b0
  __vfs_read+0x3c/0x70
  vfs_read+0xac/0x170
  ksys_read+0x7c/0x140
  system_call+0x5c/0x68

Fix it by adding a pair of rcu_read_lock/unlock() and use
cond_resched_rcu() to avoid the situation where walking of a large
number of items  may prevent scheduling for a long time.

Signed-off-by: Qian Cai <[email-masked]>
Reviewed-by: Eric Dumazet <[email-masked]>
Signed-off-by: David S. Miller <[email-masked]>
---
 net/ipv4/fib_trie.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
index 195469a13371..85a44099b7c3 100644
--- a/net/ipv4/fib_trie.c
+++ b/net/ipv4/fib_trie.c
@@ -2473,6 +2473,7 @@ static int fib_triestat_seq_show(struct seq_file *seq, void *v)
 		   " %zd bytes, size of tnode: %zd bytes.\n",
 		   LEAF_SIZE, TNODE_SIZE(0));
 
+	rcu_read_lock();
 	for (h = 0; h < FIB_TABLE_HASHSZ; h++) {
 		struct hlist_head *head = &net->ipv4.fib_table_hash[h];
 		struct fib_table *tb;
@@ -2492,7 +2493,9 @@ static int fib_triestat_seq_show(struct seq_file *seq, void *v)
 			trie_show_usage(seq, t->stats);
 #endif
 		}
+		cond_resched_rcu();
 	}
+	rcu_read_unlock();
 
 	return 0;
 }
-- 
2.20.1