linux-kernel - [PATCH 2.6.32 12/38] [PATCH 12/38] net: Fix skb_set

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Date:	Sun, 29 Nov 2015 22:47:14 +0100
From:	Willy Tarreau <w@....eu>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	Brenden Blanco <bblanco@...mgrid.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	"David S. Miller" <davem@...emloft.net>,
	Ben Hutchings <ben@...adent.org.uk>, Willy Tarreau <w@....eu>
Subject: [PATCH 2.6.32 12/38] [PATCH 12/38] net: Fix skb_set_peeked use-after-free bug

2.6.32-longterm review patch.  If anyone has any objections, please let me know.

------------------

commit a0a2a6602496a45ae838a96db8b8173794b5d398 upstream.

The commit 738ac1ebb96d02e0d23bc320302a6ea94c612dec ("net: Clone
skb before setting peeked flag") introduced a use-after-free bug
in skb_recv_datagram.  This is because skb_set_peeked may create
a new skb and free the existing one.  As it stands the caller will
continue to use the old freed skb.

This patch fixes it by making skb_set_peeked return the new skb
(or the old one if unchanged).

Fixes: 738ac1ebb96d ("net: Clone skb before setting peeked flag")
Reported-by: Brenden Blanco <bblanco@...mgrid.com>
Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>
Tested-by: Brenden Blanco <bblanco@...mgrid.com>
Reviewed-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
(cherry picked from commit e553622ccb6b6e06079f980f55cf04128db3420c)
Signed-off-by: Willy Tarreau <w@....eu>
---
 net/core/datagram.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/net/core/datagram.c b/net/core/datagram.c
index cbb3100..c855336 100644
--- a/net/core/datagram.c
+++ b/net/core/datagram.c
@@ -127,12 +127,12 @@ out_noerr:
 	goto out;
 }
 
-static int skb_set_peeked(struct sk_buff *skb)
+static struct sk_buff *skb_set_peeked(struct sk_buff *skb)
 {
 	struct sk_buff *nskb;
 
 	if (skb->peeked)
-		return 0;
+		return skb;
 
 	/* We have to unshare an skb before modifying it. */
 	if (!skb_shared(skb))
@@ -140,7 +140,7 @@ static int skb_set_peeked(struct sk_buff *skb)
 
 	nskb = skb_clone(skb, GFP_ATOMIC);
 	if (!nskb)
-		return -ENOMEM;
+		return ERR_PTR(-ENOMEM);
 
 	skb->prev->next = nskb;
 	skb->next->prev = nskb;
@@ -153,7 +153,7 @@ static int skb_set_peeked(struct sk_buff *skb)
 done:
 	skb->peeked = 1;
 
-	return 0;
+	return skb;
 }
 
 /**
@@ -214,8 +214,9 @@ struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned flags,
 			*peeked = skb->peeked;
 			if (flags & MSG_PEEK) {
 
-				error = skb_set_peeked(skb);
-				if (error)
+				skb = skb_set_peeked(skb);
+				error = PTR_ERR(skb);
+				if (IS_ERR(skb))
 					goto unlock_err;
 
 				atomic_inc(&skb->users);
-- 
1.7.12.2.21.g234cd45.dirty



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