l2tp: Fix potential memory corruption in pppol2tp_recvmsg()

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@netric.org>
Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 12 insertions, 8 deletions

diff --git a/drivers/net/pppol2tp.c b/drivers/net/pppol2tp.c
index 70cfdb46aa27..f9298827a76c 100644
--- a/drivers/net/pppol2tp.c
+++ b/drivers/net/pppol2tp.c
@@ -783,14 +783,18 @@ static int pppol2tp_recvmsg(struct kiocb *iocb, struct socket *sock,
         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;