[patch 4/9] l2tp: Fix potential memory corruption inpppol2tp_recvmsg()

[Greg KH]

2.6.25-stable review patch.  If anyone has any objections, please let us
know.

------------------
From: James Chapman <jchapman@xxxxxxxxxxx>

[ Upstream commit 6b6707a50c7598a83820077393f8823ab791abf8 ]

This patch fixes a potential memory corruption in
pppol2tp_recvmsg(). If skb->len is bigger than the caller's buffer
length, memcpy_toiovec() will go into unintialized data on the kernel
heap, interpret it as an iovec and start modifying memory.

The fix is to change the memcpy_toiovec() call to
skb_copy_datagram_iovec() so that paged packets (rare for PPPOL2TP)
are handled properly. Also check that the caller's buffer is big
enough for the data and set the MSG_TRUNC flag if it is not so.

Reported-by: Ilja <ilja@xxxxxxxxxx>
Signed-off-by: James Chapman <jchapman@xxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

---
 drivers/net/pppol2tp.c |   20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

--- a/drivers/net/pppol2tp.c
+++ b/drivers/net/pppol2tp.c
@@ -783,14 +783,18 @@ static int pppol2tp_recvmsg(struct kiocb
 	err = 0;
 	skb = skb_recv_datagram(sk, flags & ~MSG_DONTWAIT,
 				flags & MSG_DONTWAIT, &err);
-	if (skb) {
-		err = memcpy_toiovec(msg->msg_iov, (unsigned char *) skb->data,
-				     skb->len);
-		if (err < 0)
-			goto do_skb_free;
-		err = skb->len;
-	}
-do_skb_free:
+	if (!skb)
+		goto end;
+
+	if (len > skb->len)
+		len = skb->len;
+	else if (len < skb->len)
+		msg->msg_flags |= MSG_TRUNC;
+
+	err = skb_copy_datagram_iovec(skb, 0, msg->msg_iov, len);
+	if (likely(err == 0))
+		err = len;
+
 	kfree_skb(skb);
 end:
 	return err;

-- 
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