[PATCH 4.9 077/105] socket, bpf: fix possible use after free

[Greg Kroah-Hartman]

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

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

From: Eric Dumazet <edumazet@xxxxxxxxxx>


[ Upstream commit eefca20eb20c66b06cf5ed09b49b1a7caaa27b7b ]

Starting from linux-4.4, 3WHS no longer takes the listener lock.

Since this time, we might hit a use-after-free in sk_filter_charge(),
if the filter we got in the memcpy() of the listener content
just happened to be replaced by a thread changing listener BPF filter.

To fix this, we need to make sure the filter refcount is not already
zero before incrementing it again.

Fixes: e994b2f0fb92 ("tcp: do not lock listener to process SYN packets")
Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Acked-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Acked-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 net/core/filter.c |   15 +++++++++++++--
 net/core/sock.c   |    5 ++++-
 2 files changed, 17 insertions(+), 3 deletions(-)

--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -937,20 +937,31 @@ void sk_filter_uncharge(struct sock *sk,
 /* try to charge the socket memory if there is space available
  * return true on success
  */
-bool sk_filter_charge(struct sock *sk, struct sk_filter *fp)
+static bool __sk_filter_charge(struct sock *sk, struct sk_filter *fp)
 {
 	u32 filter_size = bpf_prog_size(fp->prog->len);
 
 	/* same check as in sock_kmalloc() */
 	if (filter_size <= sysctl_optmem_max &&
 	    atomic_read(&sk->sk_omem_alloc) + filter_size < sysctl_optmem_max) {
-		atomic_inc(&fp->refcnt);
 		atomic_add(filter_size, &sk->sk_omem_alloc);
 		return true;
 	}
 	return false;
 }
 
+bool sk_filter_charge(struct sock *sk, struct sk_filter *fp)
+{
+	if (!atomic_inc_not_zero(&fp->refcnt))
+		return false;
+
+	if (!__sk_filter_charge(sk, fp)) {
+		sk_filter_release(fp);
+		return false;
+	}
+	return true;
+}
+
 static struct bpf_prog *bpf_migrate_filter(struct bpf_prog *fp)
 {
 	struct sock_filter *old_prog;
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -1528,13 +1528,16 @@ struct sock *sk_clone_lock(const struct
 		sock_reset_flag(newsk, SOCK_DONE);
 		skb_queue_head_init(&newsk->sk_error_queue);
 
-		filter = rcu_dereference_protected(newsk->sk_filter, 1);
+		rcu_read_lock();
+		filter = rcu_dereference(sk->sk_filter);
 		if (filter != NULL)
 			/* though it's an empty new sock, the charging may fail
 			 * if sysctl_optmem_max was changed between creation of
 			 * original socket and cloning
 			 */
 			is_charged = sk_filter_charge(newsk, filter);
+		RCU_INIT_POINTER(newsk->sk_filter, filter);
+		rcu_read_unlock();
 
 		if (unlikely(!is_charged || xfrm_sk_clone_policy(newsk, sk))) {
 			/* We need to make sure that we don't uncharge the new