net: add limit for socket backlog (8eae939f) · Commits · 戴 / test

We got system OOM while running some UDP netperf testing on the loopback device. The case is multiple senders sent stream UDP packets to a single receiver via loopback on local host. Of course, the receiver is not able to handle all the packets in time. But we surprisingly found that these packets were not discarded due to the receiver's sk->sk_rcvbuf limit. Instead, they are kept queuing to sk->sk_backlog and finally ate up all the memory. We believe this is a secure hole that a none privileged user can crash the system. The root cause for this problem is, when the receiver is doing __release_sock() (i.e. after userspace recv, kernel udp_recvmsg -> skb_free_datagram_locked -> release_sock), it moves skbs from backlog to sk_receive_queue with the softirq enabled. In the above case, multiple busy senders will almost make it an endless loop. The skbs in the backlog end up eat all the system memory. The issue is not only for UDP. Any protocols using socket backlog is potentially affected. The patch adds limit for socket backlog so that the backlog size cannot be expanded endlessly. Reported-by:

Alex Shi <alex.shi@intel.com> Cc: David Miller <davem@davemloft.net> Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net> Cc: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru Cc: "Pekka Savola (ipv6)" <pekkas@netcore.fi> Cc: Patrick McHardy <kaber@trash.net> Cc: Vlad Yasevich <vladislav.yasevich@hp.com> Cc: Sridhar Samudrala <sri@us.ibm.com> Cc: Jon Maloy <jon.maloy@ericsson.com> Cc: Allan Stephens <allan.stephens@windriver.com> Cc: Andrew Hendry <andrew.hendry@gmail.com> Signed-off-by:

Zhu Yi <yi.zhu@intel.com> Signed-off-by:

Eric Dumazet <eric.dumazet@gmail.com> Acked-by:

Arnaldo Carvalho de Melo <acme@redhat.com> Signed-off-by:

David S. Miller <davem@davemloft.net>

include/net/sock.h

+14 −1

Original line number	Diff line number	Diff line
		@@ -253,6 +253,8 @@ struct sock {
		struct {
		struct sk_buff *head;
		struct sk_buff *tail;
		int len;
		int limit;
		} sk_backlog;
		wait_queue_head_t *sk_sleep;
		struct dst_entry *sk_dst_cache;
		@@ -589,7 +591,7 @@ static inline int sk_stream_memory_free(struct sock *sk)
		return sk->sk_wmem_queued < sk->sk_sndbuf;
		}

		/* The per-socket spinlock must be held here. */
		/* OOB backlog add */
		static inline void sk_add_backlog(struct sock sk, struct sk_buff skb)
		{
		if (!sk->sk_backlog.tail) {
		@@ -601,6 +603,17 @@ static inline void sk_add_backlog(struct sock sk, struct sk_buff skb)
		skb->next = NULL;
		}

		/* The per-socket spinlock must be held here. */
		static inline int sk_add_backlog_limited(struct sock sk, struct sk_buff skb)
		{
		if (sk->sk_backlog.len >= max(sk->sk_backlog.limit, sk->sk_rcvbuf << 1))
		return -ENOBUFS;

		sk_add_backlog(sk, skb);
		sk->sk_backlog.len += skb->truesize;
		return 0;
		}

		static inline int sk_backlog_rcv(struct sock sk, struct sk_buff skb)
		{
		return sk->sk_backlog_rcv(sk, skb);

net/core/sock.c

+14 −2

Original line number	Diff line number	Diff line
		@@ -340,8 +340,12 @@ int sk_receive_skb(struct sock sk, struct sk_buff skb, const int nested)
		rc = sk_backlog_rcv(sk, skb);

		mutex_release(&sk->sk_lock.dep_map, 1, _RET_IP_);
		} else
		sk_add_backlog(sk, skb);
		} else if (sk_add_backlog_limited(sk, skb)) {
		bh_unlock_sock(sk);
		atomic_inc(&sk->sk_drops);
		goto discard_and_relse;
		}

		bh_unlock_sock(sk);
		out:
		sock_put(sk);
		@@ -1139,6 +1143,7 @@ struct sock sk_clone(const struct sock sk, const gfp_t priority)
		sock_lock_init(newsk);
		bh_lock_sock(newsk);
		newsk->sk_backlog.head = newsk->sk_backlog.tail = NULL;
		newsk->sk_backlog.len = 0;

		atomic_set(&newsk->sk_rmem_alloc, 0);
		/*
		@@ -1542,6 +1547,12 @@ static void __release_sock(struct sock *sk)

		bh_lock_sock(sk);
		} while ((skb = sk->sk_backlog.head) != NULL);

		/*
		* Doing the zeroing here guarantee we can not loop forever
		* while a wild producer attempts to flood us.
		*/
		sk->sk_backlog.len = 0;
		}

		/**
		@@ -1874,6 +1885,7 @@ void sock_init_data(struct socket sock, struct sock sk)
		sk->sk_allocation = GFP_KERNEL;
		sk->sk_rcvbuf = sysctl_rmem_default;
		sk->sk_sndbuf = sysctl_wmem_default;
		sk->sk_backlog.limit = sk->sk_rcvbuf << 1;
		sk->sk_state = TCP_CLOSE;
		sk_set_socket(sk, sock);

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