[PATCH 3.16.y-ckt 099/104] net: add pfmemalloc check in sk_add_backlog()

[Luis Henriques]

3.16.7-ckt19 -stable review patch.  If anyone has any objections, please let me know.

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

From: Eric Dumazet <edumazet at google.com>

commit c7c49b8fde26b74277188bdc6c9dca38db6fa35b upstream.

Greg reported crashes hitting the following check in __sk_backlog_rcv()

	BUG_ON(!sock_flag(sk, SOCK_MEMALLOC));

The pfmemalloc bit is currently checked in sk_filter().

This works correctly for TCP, because sk_filter() is ran in
tcp_v[46]_rcv() before hitting the prequeue or backlog checks.

For UDP or other protocols, this does not work, because the sk_filter()
is ran from sock_queue_rcv_skb(), which might be called _after_ backlog
queuing if socket is owned by user by the time packet is processed by
softirq handler.

Fixes: b4b9e35585089 ("netvm: set PF_MEMALLOC as appropriate during SKB processing")
Signed-off-by: Eric Dumazet <edumazet at google.com>
Reported-by: Greg Thelen <gthelen at google.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 include/net/sock.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/include/net/sock.h b/include/net/sock.h
index 927cfc56b76a..9c24377a7bc9 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -807,6 +807,14 @@ static inline __must_check int sk_add_backlog(struct sock *sk, struct sk_buff *s
 	if (sk_rcvqueues_full(sk, skb, limit))
 		return -ENOBUFS;
 
+	/*
+	 * If the skb was allocated from pfmemalloc reserves, only
+	 * allow SOCK_MEMALLOC sockets to use it as this socket is
+	 * helping free memory
+	 */
+	if (skb_pfmemalloc(skb) && !sock_flag(sk, SOCK_MEMALLOC))
+		return -ENOMEM;
+
 	__sk_add_backlog(sk, skb);
 	sk->sk_backlog.len += skb->truesize;
 	return 0;