[PATCH v1 net 1/3] af_unix: Update last skb marker in manage_oob().

[Kuniyuki Iwashima <kuniyu@google.com>]

Fahad Alharbi reported that blocking recv(MSG_PEEK) could hog CPU
due to OOB skb.

In the following cases, manage_oob() skips OOB skb(s) and returns
NULL for the last recv(MSG_PEEK):

  socketpair(AF_UNIX, SOCK_STREAM, 0, sk);

  1) skb -> OOB skb -> NULL
     send(sk[0], "ab", 2, MSG_OOB);
     recv(sk[1], buf, 0, MSG_PEEK);

  2) skb -> consumed OOB skb -> NULL
     send(sk[0], "ab", 2, MSG_OOB);
     recv(sk[1], buf, 1, MSG_OOB);
     recv(sk[1], buf, 0, MSG_PEEK);

  3) consumed OOB skb -> OOB skb -> NULL
     send(sk[0], "a", 1, MSG_OOB);
     recv(sk[1], buf, 0, MSG_OOB);
     send(sk[0], "b", 1, MSG_OOB);
     recv(sk[1], buf, 1, MSG_PEEK);

Then, @copied is 0 in unix_stream_read_generic() (zero-length buffer,
or non-OOB skb is not yet consumed), and unix_stream_data_wait() is
called.

However, it returns immediately because @last is not updated in
unix_stream_read_generic(), and the thread busy-waits for a new skb.

Let's update @last in manage_oob().

For MSG_PEEK, @last is updated with the skipped OOB, and for the
non-peek case, @last matches the returned value (when !copied)
because OOB is unlinked.

Note that manage_oob() is inlined and no stack canary is added.

Fixes: 22dd70eb2c3d ("af_unix: Don't peek OOB data without MSG_OOB.")
Reported-by: Fahad Alharbi <fahad@codepure.com>
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 net/unix/af_unix.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 13f9926bf205..6861370062df 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -2812,8 +2812,8 @@ static int unix_stream_recv_urg(struct unix_stream_read_state *state)
 	return 1;
 }
 
-static struct sk_buff *manage_oob(struct sk_buff *skb, struct sock *sk,
-				  int flags, int copied)
+static struct sk_buff *manage_oob(struct sk_buff *skb, struct sk_buff **last,
+				  struct sock *sk, int flags, int copied)
 {
 	struct sk_buff *read_skb = NULL, *unread_skb = NULL;
 	struct unix_sock *u = unix_sk(sk);
@@ -2827,11 +2827,13 @@ static struct sk_buff *manage_oob(struct sk_buff *skb, struct sock *sk,
 		if (copied && (!u->oob_skb || skb == u->oob_skb)) {
 			skb = NULL;
 		} else if (flags & MSG_PEEK) {
+			*last = skb;
 			skb = skb_peek_next(skb, &sk->sk_receive_queue);
 		} else {
 			read_skb = skb;
 			skb = skb_peek_next(skb, &sk->sk_receive_queue);
 			__skb_unlink(read_skb, &sk->sk_receive_queue);
+			*last = skb;
 		}
 
 		if (!skb)
@@ -2850,8 +2852,10 @@ static struct sk_buff *manage_oob(struct sk_buff *skb, struct sock *sk,
 			__skb_unlink(skb, &sk->sk_receive_queue);
 			unread_skb = skb;
 			skb = skb_peek(&sk->sk_receive_queue);
+			*last = skb;
 		}
 	} else if (!sock_flag(sk, SOCK_URGINLINE)) {
+		*last = skb;
 		skb = skb_peek_next(skb, &sk->sk_receive_queue);
 	}
 
@@ -2971,7 +2975,7 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state,
 again:
 #if IS_ENABLED(CONFIG_AF_UNIX_OOB)
 		if (skb) {
-			skb = manage_oob(skb, sk, flags, copied);
+			skb = manage_oob(skb, &last, sk, flags, copied);
 			if (!skb && copied) {
 				unix_state_unlock(sk);
 				break;
-- 
2.55.0.970.g62bdec98f9-goog