From: John Fastabend <john.fastabend@gmail.com>
Date: Thu, 21 Nov 2019 08:25:09 -0800
Subject: bpf: skmsg, fix potential psock NULL pointer dereference
Patch-mainline: v5.5-rc1
Git-commit: 8163999db445021f2651a8a47b5632483e8722ea
References: bsc#1109837

Report from Dan Carpenter,

 net/core/skmsg.c:792 sk_psock_write_space()
 error: we previously assumed 'psock' could be null (see line 790)

 net/core/skmsg.c
   789 psock = sk_psock(sk);
   790 if (likely(psock && sk_psock_test_state(psock, SK_PSOCK_TX_ENABLED)))
 Check for NULL
   791 schedule_work(&psock->work);
   792 write_space = psock->saved_write_space;
                     ^^^^^^^^^^^^^^^^^^^^^^^^
   793          rcu_read_unlock();
   794          write_space(sk);

Ensure psock dereference on line 792 only occurs if psock is not null.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 604326b41a6f ("bpf, sockmap: convert to generic sk_msg interface")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
---
 net/core/skmsg.c |   13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

--- a/net/core/skmsg.c
+++ b/net/core/skmsg.c
@@ -757,15 +757,18 @@ static void sk_psock_strp_data_ready(str
 static void sk_psock_write_space(struct sock *sk)
 {
 	struct sk_psock *psock;
-	void (*write_space)(struct sock *sk);
+	void (*write_space)(struct sock *sk) = NULL;
 
 	rcu_read_lock();
 	psock = sk_psock(sk);
-	if (likely(psock && sk_psock_test_state(psock, SK_PSOCK_TX_ENABLED)))
-		schedule_work(&psock->work);
-	write_space = psock->saved_write_space;
+	if (likely(psock)) {
+		if (sk_psock_test_state(psock, SK_PSOCK_TX_ENABLED))
+			schedule_work(&psock->work);
+		write_space = psock->saved_write_space;
+	}
 	rcu_read_unlock();
-	write_space(sk);
+	if (write_space)
+		write_space(sk);
 }
 
 int sk_psock_init_strp(struct sock *sk, struct sk_psock *psock)