CVE-2025-21925: llc: do not use skb_get() before dev_queue_xmit()

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

llc: do not use skbget() before devqueuexmit()

syzbot is able to crash hosts [1], using llc and devices not supporting IFFTXSKBSHARING.

In this case, e1000 driver calls ethskbpad(), while the skb is shared.

Simply replace skbget() by skbclone() in net/llc/llcsac.c

Note that e1000 driver might have an issue with pktgen, because it does not clear IFFTXSKBSHARING, this is an orthogonal change.

We need to audit other skbget() uses in net/llc.

[1]

kernel BUG at net/core/skbuff.c:2178 ! Oops: invalid opcode: 0000 [#1] PREEMPT SMP KASAN NOPTI CPU: 0 UID: 0 PID: 16371 Comm: syz.2.2764 Not tainted 6.14.0-rc4-syzkaller-00052-gac9c34d1e45a #0 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014 RIP: 0010:pskbexpandhead+0x6ce/0x1240 net/core/skbuff.c:2178 Call Trace: <TASK> skbpad+0x18a/0x610 net/core/skbuff.c:2466 skbputpadto include/linux/skbuff.h:3843 [inline] skbputpadto include/linux/skbuff.h:3862 [inline] ethskbpad include/linux/etherdevice.h:656 [inline] e1000xmitframe+0x2d99/0x5800 drivers/net/ethernet/intel/e1000/e1000main.c:3128 netdevstartxmit include/linux/netdevice.h:5151 [inline] netdevstartxmit include/linux/netdevice.h:5160 [inline] xmitone net/core/dev.c:3806 [inline] devhardstartxmit+0x9a/0x7b0 net/core/dev.c:3822 schdirectxmit+0x1ae/0xc30 net/sched/schgeneric.c:343 devxmitskb net/core/dev.c:4045 [inline] devqueuexmit+0x13d4/0x43e0 net/core/dev.c:4621 devqueuexmit include/linux/netdevice.h:3313 [inline] llcsapactionsendtestc+0x268/0x320 net/llc/llcsac.c:144 llcexecsaptransactions net/llc/llcsap.c:153 [inline] llcsapnextstate net/llc/llcsap.c:182 [inline] llcsapstateprocess+0x239/0x510 net/llc/llcsap.c:209 llcuisendmsg+0xd0d/0x14e0 net/llc/afllc.c:993 socksendmsgnosec net/socket.c:718 [inline]