linux-kernel - [PATCH net] af_unix: fix garbage collect vs. MSG

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Date:   Mon, 26 Jul 2021 17:36:21 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     netdev@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        Miklos Szeredi <mszeredi@...hat.com>, stable@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: [PATCH net] af_unix: fix garbage collect vs. MSG_PEEK

From: Miklos Szeredi <mszeredi@...hat.com>

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
unix_gc_lock.

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 <mszeredi@...hat.com>
Cc: <stable@...r.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 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:
	https://lore.kernel.org/netdev/CAOssrKcfncAYsQWkfLGFgoOxAQJVT2hYVWdBA6Cw7hhO8RJ_wQ@mail.gmail.com/
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);
 
-- 
2.32.0