netrom: check buffer length before accessing it

In the Linux kernel, the following vulnerability has been resolved:

netrom: check buffer length before accessing it

Syzkaller reports an uninit value read from ax25cmp when sending raw message through ieee802154 implementation.

===================================================== BUG: KMSAN: uninit-value in ax25cmp+0x3a5/0x460 net/ax25/ax25_addr.c:119 ax25cmp+0x3a5/0x460 net/ax25/ax25_addr.c:119 nr_dev_get+0x20e/0x450 net/netrom/nr_route.c:601 nr_route_frame+0x1a2/0xfc0 net/netrom/nr_route.c:774 nr_xmit+0x5a/0x1c0 net/netrom/nr_dev.c:144 __netdev_start_xmit include/linux/netdevice.h:4940 [inline] netdev_start_xmit include/linux/netdevice.h:4954 [inline] xmit_one net/core/dev.c:3548 [inline] dev_hard_start_xmit+0x247/0xa10 net/core/dev.c:3564 __dev_queue_xmit+0x33b8/0x5130 net/core/dev.c:4349 dev_queue_xmit include/linux/netdevice.h:3134 [inline] raw_sendmsg+0x654/0xc10 net/ieee802154/socket.c:299 ieee802154_sock_sendmsg+0x91/0xc0 net/ieee802154/socket.c:96 sock_sendmsg_nosec net/socket.c:730 [inline] __sock_sendmsg net/socket.c:745 [inline] ____sys_sendmsg+0x9c2/0xd60 net/socket.c:2584 ___sys_sendmsg+0x28d/0x3c0 net/socket.c:2638 __sys_sendmsg net/socket.c:2667 [inline] __do_sys_sendmsg net/socket.c:2676 [inline] __se_sys_sendmsg net/socket.c:2674 [inline] __x64_sys_sendmsg+0x307/0x490 net/socket.c:2674 do_syscall_x64 arch/x86/entry/common.c:52 [inline] do_syscall_64+0x44/0x110 arch/x86/entry/common.c:83 entry_SYSCALL_64_after_hwframe+0x63/0x6b

Uninit was created at: slab_post_alloc_hook+0x129/0xa70 mm/slab.h:768 slab_alloc_node mm/slub.c:3478 [inline] kmem_cache_alloc_node+0x5e9/0xb10 mm/slub.c:3523 kmalloc_reserve+0x13d/0x4a0 net/core/skbuff.c:560 __alloc_skb+0x318/0x740 net/core/skbuff.c:651 alloc_skb include/linux/skbuff.h:1286 [inline] alloc_skb_with_frags+0xc8/0xbd0 net/core/skbuff.c:6334 sock_alloc_send_pskb+0xa80/0xbf0 net/core/sock.c:2780 sock_alloc_send_skb include/net/sock.h:1884 [inline] raw_sendmsg+0x36d/0xc10 net/ieee802154/socket.c:282 ieee802154_sock_sendmsg+0x91/0xc0 net/ieee802154/socket.c:96 sock_sendmsg_nosec net/socket.c:730 [inline] __sock_sendmsg net/socket.c:745 [inline] ____sys_sendmsg+0x9c2/0xd60 net/socket.c:2584 ___sys_sendmsg+0x28d/0x3c0 net/socket.c:2638 __sys_sendmsg net/socket.c:2667 [inline] __do_sys_sendmsg net/socket.c:2676 [inline] __se_sys_sendmsg net/socket.c:2674 [inline] __x64_sys_sendmsg+0x307/0x490 net/socket.c:2674 do_syscall_x64 arch/x86/entry/common.c:52 [inline] do_syscall_64+0x44/0x110 arch/x86/entry/common.c:83 entry_SYSCALL_64_after_hwframe+0x63/0x6b

CPU: 0 PID: 5037 Comm: syz-executor166 Not tainted 6.7.0-rc7-syzkaller-00003-gfbafc3e621c3 #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/17/2023 =====================================================

This issue occurs because the skb buffer is too small, and it's actual allocation is aligned. This hides an actual issue, which is that nr_route_frame does not validate the buffer size before using it.

Fix this issue by checking skb->len before accessing any fields in skb->data.

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.