pptp: ensure minimal skb length in pptp_xmit()

Commit aabc659 ("net: ppp: Add bound checking for skb data
on ppp_sync_txmung") fixed ppp_sync_txmunge()

We need a similar fix in pptp_xmit(), otherwise we might
read uninit data as reported by syzbot.

BUG: KMSAN: uninit-value in pptp_xmit+0xc34/0x2720 drivers/net/ppp/pptp.c:193
  pptp_xmit+0xc34/0x2720 drivers/net/ppp/pptp.c:193
  ppp_channel_bridge_input drivers/net/ppp/ppp_generic.c:2290 [inline]
  ppp_input+0x1d6/0xe60 drivers/net/ppp/ppp_generic.c:2314
  pppoe_rcv_core+0x1e8/0x760 drivers/net/ppp/pppoe.c:379
  sk_backlog_rcv+0x142/0x420 include/net/sock.h:1148
  __release_sock+0x1d3/0x330 net/core/sock.c:3213
  release_sock+0x6b/0x270 net/core/sock.c:3767
  pppoe_sendmsg+0x15d/0xcb0 drivers/net/ppp/pppoe.c:904
  sock_sendmsg_nosec net/socket.c:712 [inline]
  __sock_sendmsg+0x330/0x3d0 net/socket.c:727
  ____sys_sendmsg+0x893/0xd80 net/socket.c:2566
  ___sys_sendmsg+0x271/0x3b0 net/socket.c:2620
  __sys_sendmmsg+0x2d9/0x7c0 net/socket.c:2709

Fixes: 1da177e ("Linux-2.6.12-rc2")
Reported-by: syzbot+afad90ffc8645324afe5@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/68887d86.a00a0220.b12ec.00cd.GAE@google.com/T/#u
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Dawid Osuchowski <dawid.osuchowski@linux.intel.com>
Link: https://patch.msgid.link/20250729080207.1863408-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

drivers/net/ppp/pptp.c

@@ -159,9 +159,7 @@ static int pptp_xmit(struct ppp_channel *chan, struct sk_buff *skb)
 	int len;
 	unsigned char *data;
 	__u32 seq_recv;
-
-
-	struct rtable *rt;
+	struct rtable *rt = NULL;
 	struct net_device *tdev;
 	struct iphdr  *iph;
 	int    max_headroom;
@@ -179,16 +177,20 @@ static int pptp_xmit(struct ppp_channel *chan, struct sk_buff *skb)
 
 	if (skb_headroom(skb) < max_headroom || skb_cloned(skb) || skb_shared(skb)) {
 		struct sk_buff *new_skb = skb_realloc_headroom(skb, max_headroom);
-		if (!new_skb) {
-			ip_rt_put(rt);
+
+		if (!new_skb)
 			goto tx_error;
-		}
+
 		if (skb->sk)
 			skb_set_owner_w(new_skb, skb->sk);
 		consume_skb(skb);
 		skb = new_skb;
 	}
 
+	/* Ensure we can safely access protocol field and LCP code */
+	if (!pskb_may_pull(skb, 3))
+		goto tx_error;
+
 	data = skb->data;
 	islcp = ((data[0] << 8) + data[1]) == PPP_LCP && 1 <= data[2] && data[2] <= 7;
 
@@ -262,6 +264,7 @@ static int pptp_xmit(struct ppp_channel *chan, struct sk_buff *skb)
 	return 1;
 
 tx_error:
+	ip_rt_put(rt);
 	kfree_skb(skb);
 	return 1;
 }