pppoe: Fix memory leak in pppoe_sendmsg()

syzbot reports a memory leak in pppoe_sendmsg [1].

The problem is in the pppoe_recvmsg() function that handles errors
in the wrong order. For the skb_recv_datagram() function, check
the pointer to skb for NULL first, and then check the 'error' variable,
because the skb_recv_datagram() function can set 'error'
to -EAGAIN in a loop but return a correct pointer to socket buffer
after a number of attempts, though 'error' remains set to -EAGAIN.

skb_recv_datagram
      __skb_recv_datagram          // Loop. if (err == -EAGAIN) then
                                   // go to the next loop iteration
          __skb_try_recv_datagram  // if (skb != NULL) then return 'skb'
                                   // else if a signal is received then
                                   // return -EAGAIN

Found by InfoTeCS on behalf of Linux Verification Center
(linuxtesting.org) with Syzkaller.

Link: https://syzkaller.appspot.com/bug?extid=6bdfd184eac7709e5cc9
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: syzbot+6bdfd184eac7709e5cc9@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=6bdfd184eac7709e5cc9
Signed-off-by: Gavrilov Ilia <Ilia.Gavrilov@infotecs.ru>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Link: https://lore.kernel.org/r/20240214085814.3894917-1-Ilia.Gavrilov@infotecs.ru
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/drivers/net/ppp/pppoe.c b/drivers/net/ppp/pppoe.c
index 8e7238e97d0a71708ebcddda9b1e1a50ab28c17d..2ea4f4890d23b5f1c5229c7f8b303ee85a954037 100644 (file)
--- a/drivers/net/ppp/pppoe.c
+++ b/drivers/net/ppp/pppoe.c
@@ -1007,26 +1007,21 @@ static int pppoe_recvmsg(struct socket *sock, struct msghdr *m,
        struct sk_buff *skb;
        int error = 0;
 
-       if (sk->sk_state & PPPOX_BOUND) {
-               error = -EIO;
-               goto end;
-       }
+       if (sk->sk_state & PPPOX_BOUND)
+               return -EIO;
 
        skb = skb_recv_datagram(sk, flags, &error);
-       if (error < 0)
-               goto end;
+       if (!skb)
+               return error;
 
-       if (skb) {
-               total_len = min_t(size_t, total_len, skb->len);
-               error = skb_copy_datagram_msg(skb, 0, m, total_len);
-               if (error == 0) {
-                       consume_skb(skb);
-                       return total_len;
-               }
+       total_len = min_t(size_t, total_len, skb->len);
+       error = skb_copy_datagram_msg(skb, 0, m, total_len);
+       if (error == 0) {
+               consume_skb(skb);
+               return total_len;
        }
 
        kfree_skb(skb);
-end:
        return error;
 }