net: add pfmemalloc check in sk_add_backlog()

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@google.com>
Reported-by: Greg Thelen <gthelen@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 8 insertions, 0 deletions

diff --git a/include/net/sock.h b/include/net/sock.h
index 7aa78440559a..e23717013a4e 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -828,6 +828,14 @@ static inline __must_check int sk_add_backlog(struct sock *sk, struct sk_buff *s
         if (sk_rcvqueues_full(sk, 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;