pptp: ensure minimal skb length in pptp_xmit()

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: pptp: ensure minimal skb length in pptp_xmit() Commit aabc6596ffb3 ("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

References

• https://git.kernel.org/stable/c/97b8c5d322c5c0038cac4bc56fdbe237d0be426f
• https://git.kernel.org/stable/c/26672f1679b143aa34fca0b6046b7fd0c184770d
• https://git.kernel.org/stable/c/5de7513f38f3c19c0610294ee478242bea356f8c
• https://git.kernel.org/stable/c/504cc4ab91073d2ac7404ad146139f86ecee7193
• https://git.kernel.org/stable/c/5005d24377378a20e5c0e53052fc4ebdcdcbc611
• https://git.kernel.org/stable/c/ea99b88b1999ebcb24d5d3a6b7910030f40d3bba
• https://git.kernel.org/stable/c/b7dcda76fd0615c0599c89f36873a6cd48e02dbb
• https://git.kernel.org/stable/c/1a04db0fd75cb6034fc27a56b67b3b8b9022a98c
• https://git.kernel.org/stable/c/de9c4861fb42f0cd72da844c3c34f692d5895b7b