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From: Greg Kroah-Hartman <>
Cc:,,, Miklos Szeredi <>,,
	Greg Kroah-Hartman <>
Subject: [PATCH net] af_unix: fix garbage collect vs. MSG_PEEK
Date: Mon, 26 Jul 2021 17:36:21 +0200	[thread overview]
Message-ID: <> (raw)

From: Miklos Szeredi <>

Gc assumes that in-flight sockets that don't have an external ref can't
gain one while unix_gc_lock is held.  That is true because
unix_notinflight() will be called before detaching fds, which takes

Only MSG_PEEK was somehow overlooked.  That one also clones the fds, also
keeping them in the skb.  But through MSG_PEEK an external reference can
definitely be gained without ever touching unix_gc_lock.

Signed-off-by: Miklos Szeredi <>
Cc: <>
Signed-off-by: Greg Kroah-Hartman <>
 net/unix/af_unix.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

Note, this is a resend of this old submission that somehow fell through
the cracks:
and was never submitted "properly" and this issue never seemed to get
resolved properly.

I've cleaned it up and made the change much smaller and localized to
only one file.  I kept Miklos's authorship as he did the hard work on
this, I just removed lines and fixed a formatting issue :)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 23c92ad15c61..cdea997aa5bf 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1526,6 +1526,18 @@ static int unix_getname(struct socket *sock, struct sockaddr *uaddr, int peer)
 	return err;
+static void unix_peek_fds(struct scm_cookie *scm, struct sk_buff *skb)
+	scm->fp = scm_fp_dup(UNIXCB(skb).fp);
+	/* During garbage collection it is assumed that in-flight sockets don't
+	 * get a new external reference.  So we need to wait until current run
+	 * finishes.
+	 */
+	spin_lock(&unix_gc_lock);
+	spin_unlock(&unix_gc_lock);
 static int unix_scm_to_skb(struct scm_cookie *scm, struct sk_buff *skb, bool send_fds)
 	int err = 0;
@@ -2175,7 +2187,7 @@ static int unix_dgram_recvmsg(struct socket *sock, struct msghdr *msg,
 		sk_peek_offset_fwd(sk, size);
 		if (UNIXCB(skb).fp)
-			scm.fp = scm_fp_dup(UNIXCB(skb).fp);
+			unix_peek_fds(&scm, skb);
 	err = (flags & MSG_TRUNC) ? skb->len - skip : size;
@@ -2418,7 +2430,7 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state,
 			/* It is questionable, see note in unix_dgram_recvmsg.
 			if (UNIXCB(skb).fp)
-				scm.fp = scm_fp_dup(UNIXCB(skb).fp);
+				unix_peek_fds(&scm, skb);
 			sk_peek_offset_fwd(sk, chunk);

             reply	other threads:[~2021-07-26 15:38 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-26 15:36 Greg Kroah-Hartman [this message]
2021-07-26 19:27 ` [PATCH net] af_unix: fix garbage collect vs. MSG_PEEK Kees Cook
2021-07-29 14:29   ` Miklos Szeredi

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