netdev - Re: [PATCH -next v2] unix stream: Fix use-after-free crashes

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Message-ID: <1315465919.2532.19.camel@edumazet-laptop>
Date:	Thu, 08 Sep 2011 09:11:59 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Yan, Zheng" <zheng.z.yan@...el.com>
Cc:	Tim Chen <tim.c.chen@...ux.intel.com>,
	"sedat.dilek@...il.com" <sedat.dilek@...il.com>,
	"Yan, Zheng" <yanzheng@...n.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"sfr@...b.auug.org.au" <sfr@...b.auug.org.au>,
	"jirislaby@...il.com" <jirislaby@...il.com>,
	"Shi, Alex" <alex.shi@...el.com>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>
Subject: Re: [PATCH -next v2] unix stream: Fix use-after-free crashes

Le jeudi 08 septembre 2011 à 14:22 +0800, Yan, Zheng a écrit :

> I don't think so. unix_scm_to_skb() calls unix_attach_fds(), it
> always duplicates scm->fp. 

What a mess. This code is a nightmare.

Part of the mess comes from scm_destroy() and scm_release() duplication.

We should have scm_destroy() only, as before, and NULLify scm->pid/cred
in unix_scm_to_skb() when we steal references.

It makes more sense and keeps things clearer.


 include/net/scm.h  |    9 ---------
 net/unix/af_unix.c |   27 +++++++++++++++------------
 2 files changed, 15 insertions(+), 21 deletions(-)


diff --git a/include/net/scm.h b/include/net/scm.h
index 68e1e48..2a5b42f 100644
--- a/include/net/scm.h
+++ b/include/net/scm.h
@@ -78,15 +78,6 @@ static __inline__ void scm_destroy(struct scm_cookie *scm)
 		__scm_destroy(scm);
 }
 
-static __inline__ void scm_release(struct scm_cookie *scm)
-{
-	/* keep ref on pid and cred */
-	scm->pid = NULL;
-	scm->cred = NULL;
-	if (scm->fp)
-		__scm_destroy(scm);
-}
-
 static __inline__ int scm_send(struct socket *sock, struct msghdr *msg,
 			       struct scm_cookie *scm)
 {
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index e6d9d10..1fa270a 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1379,15 +1379,18 @@ static int unix_attach_fds(struct scm_cookie *scm, struct sk_buff *skb)
 }
 
 static int unix_scm_to_skb(struct scm_cookie *scm, struct sk_buff *skb,
-			   bool send_fds, bool ref)
+			   bool send_fds, bool steal_refs)
 {
 	int err = 0;
-	if (ref) {
+
+	if (!steal_refs) {
 		UNIXCB(skb).pid  = get_pid(scm->pid);
 		UNIXCB(skb).cred = get_cred(scm->cred);
 	} else {
 		UNIXCB(skb).pid  = scm->pid;
 		UNIXCB(skb).cred = scm->cred;
+		scm->pid = NULL;
+		scm->cred = NULL;
 	}
 	UNIXCB(skb).fp = NULL;
 	if (scm->fp && send_fds)
@@ -1454,7 +1457,7 @@ static int unix_dgram_sendmsg(struct kiocb *kiocb, struct socket *sock,
 	if (skb == NULL)
 		goto out;
 
-	err = unix_scm_to_skb(siocb->scm, skb, true, false);
+	err = unix_scm_to_skb(siocb->scm, skb, true, true);
 	if (err < 0)
 		goto out_free;
 	max_level = err + 1;
@@ -1550,7 +1553,7 @@ restart:
 	unix_state_unlock(other);
 	other->sk_data_ready(other, len);
 	sock_put(other);
-	scm_release(siocb->scm);
+	scm_destroy(siocb->scm);
 	return len;
 
 out_unlock:
@@ -1577,6 +1580,7 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
 	int sent = 0;
 	struct scm_cookie tmp_scm;
 	bool fds_sent = false;
+	bool steal_refs = false;
 	int max_level;
 
 	if (NULL == siocb->scm)
@@ -1638,8 +1642,11 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
 		size = min_t(int, size, skb_tailroom(skb));
 
 
-		/* Only send the fds and no ref to pid in the first buffer */
-		err = unix_scm_to_skb(siocb->scm, skb, !fds_sent, fds_sent);
+		/* Only send the fds in first buffer
+		 * Last buffer can steal our references to pid/cred
+		 */
+		steal_refs = (sent + size >= len);
+		err = unix_scm_to_skb(siocb->scm, skb, !fds_sent, steal_refs);
 		if (err < 0) {
 			kfree_skb(skb);
 			goto out;
@@ -1667,10 +1674,7 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
 		sent += size;
 	}
 
-	if (skb)
-		scm_release(siocb->scm);
-	else
-		scm_destroy(siocb->scm);
+	scm_destroy(siocb->scm);
 	siocb->scm = NULL;
 
 	return sent;
@@ -1683,8 +1687,7 @@ pipe_err:
 		send_sig(SIGPIPE, current, 0);
 	err = -EPIPE;
 out_err:
-	if (skb == NULL)
-		scm_destroy(siocb->scm);
+	scm_destroy(siocb->scm);
 out:
 	siocb->scm = NULL;
 	return sent ? : err;


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