ipv4: irq safe sk_dst_[re]set() and ipv4_sk_update_pmtu() fix

We have two different ways to handle changes to sk->sk_dst

First way (used by TCP) assumes socket lock is owned by caller, and use
no extra lock : __sk_dst_set() & __sk_dst_reset()

Another way (used by UDP) uses sk_dst_lock because socket lock is not
always taken. Note that sk_dst_lock is not softirq safe.

These ways are not inter changeable for a given socket type.

ipv4_sk_update_pmtu(), added in linux-3.8, added a race, as it used
the socket lock as synchronization, but users might be UDP sockets.

Instead of converting sk_dst_lock to a softirq safe version, use xchg()
as we did for sk_rx_dst in commit e47eb5dfb296b ("udp: ipv4: do not use
sk_dst_lock from softirq context")

In a follow up patch, we probably can remove sk_dst_lock, as it is
only used in IPv6.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Fixes: 9cb3a50c5f63e ("ipv4: Invalidate the socket cached route on pmtu events if possible")
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 6 deletions

diff --git a/include/net/sock.h b/include/net/sock.h
index 173cae485de1..c556fd9b05ac 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1768,9 +1768,11 @@ __sk_dst_set(struct sock *sk, struct dst_entry *dst)
 static inline void
 sk_dst_set(struct sock *sk, struct dst_entry *dst)
 {
-        spin_lock(&sk->sk_dst_lock);
-        __sk_dst_set(sk, dst);
-        spin_unlock(&sk->sk_dst_lock);
+        struct dst_entry *old_dst;
+
+        sk_tx_queue_clear(sk);
+        old_dst = xchg(&sk->sk_dst_cache, dst);
+        dst_release(old_dst);
 }
 
 static inline void
@@ -1782,9 +1784,7 @@ __sk_dst_reset(struct sock *sk)
 static inline void
 sk_dst_reset(struct sock *sk)
 {
-        spin_lock(&sk->sk_dst_lock);
-        __sk_dst_reset(sk);
-        spin_unlock(&sk->sk_dst_lock);
+        sk_dst_set(sk, NULL);
 }
 
 struct dst_entry *__sk_dst_check(struct sock *sk, u32 cookie);