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 pppsynctxmung") fixed pppsynctxmunge()
We need a similar fix in pptp_xmit(), otherwise we might read uninit data as reported by syzbot.
BUG: KMSAN: uninit-value in pptpxmit+0xc34/0x2720 drivers/net/ppp/pptp.c:193 pptpxmit+0xc34/0x2720 drivers/net/ppp/pptp.c:193 pppchannelbridgeinput drivers/net/ppp/pppgeneric.c:2290 [inline] pppinput+0x1d6/0xe60 drivers/net/ppp/pppgeneric.c:2314 pppoercvcore+0x1e8/0x760 drivers/net/ppp/pppoe.c:379 skbacklogrcv+0x142/0x420 include/net/sock.h:1148 releasesock+0x1d3/0x330 net/core/sock.c:3213 releasesock+0x6b/0x270 net/core/sock.c:3767 pppoesendmsg+0x15d/0xcb0 drivers/net/ppp/pppoe.c:904 socksendmsgnosec net/socket.c:712 [inline] _socksendmsg+0x330/0x3d0 net/socket.c:727 _syssendmsg+0x893/0xd80 net/socket.c:2566 _syssendmsg+0x271/0x3b0 net/socket.c:2620 _syssendmmsg+0x2d9/0x7c0 net/socket.c:2709