From 94fd4debd2e3a69cf93e766c8b328a810c228119 Mon Sep 17 00:00:00 2001 From: Kuniyuki Iwashima Date: Wed, 2 Sep 2026 20:21:50 +0000 Subject: [PATCH] af_unix: Update last skb marker in manage_oob(). 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 Signed-off-by: Kuniyuki Iwashima Link: https://patch.msgid.link/20260902202202.892676-2-kuniyu@google.com Signed-off-by: Jakub Kicinski --- 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;