llc: do not use skb_get() before dev_queue_xmit()

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

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

Simply replace skb_get() by skb_clone() in net/llc/llc_s_ac.c

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

We need to audit other skb_get() 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:pskb_expand_head+0x6ce/0x1240 net/core/skbuff.c:2178
Call Trace:
<TASK>
__skb_pad+0x18a/0x610 net/core/skbuff.c:2466
__skb_put_padto include/linux/skbuff.h:3843 [inline]
skb_put_padto include/linux/skbuff.h:3862 [inline]
eth_skb_pad include/linux/etherdevice.h:656 [inline]
e1000_xmit_frame+0x2d99/0x5800 drivers/net/ethernet/intel/e1000/e1000_main.c:3128
__netdev_start_xmit include/linux/netdevice.h:5151 [inline]
netdev_start_xmit include/linux/netdevice.h:5160 [inline]
xmit_one net/core/dev.c:3806 [inline]
dev_hard_start_xmit+0x9a/0x7b0 net/core/dev.c:3822
sch_direct_xmit+0x1ae/0xc30 net/sched/sch_generic.c:343
__dev_xmit_skb net/core/dev.c:4045 [inline]
__dev_queue_xmit+0x13d4/0x43e0 net/core/dev.c:4621
dev_queue_xmit include/linux/netdevice.h:3313 [inline]
llc_sap_action_send_test_c+0x268/0x320 net/llc/llc_s_ac.c:144
llc_exec_sap_trans_actions net/llc/llc_sap.c:153 [inline]
llc_sap_next_state net/llc/llc_sap.c:182 [inline]
llc_sap_state_process+0x239/0x510 net/llc/llc_sap.c:209
llc_ui_sendmsg+0xd0d/0x14e0 net/llc/af_llc.c:993
sock_sendmsg_nosec net/socket.c:718 [inline]

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: [e-mail]
Closes: https://lore.kernel.org/netdev/[e-mail]/T/#u
Signed-off-by: Eric Dumazet <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/llc/llc_s_ac.c

···
 #include <net/llc_s_ac.h>
 #include <net/llc_s_ev.h>
 #include <net/llc_sap.h>
-
+#include <net/sock.h>
 
 /**
  *	llc_sap_action_unitdata_ind - forward UI PDU to network layer
···
 	return 0;
 }
 
+static int llc_prepare_and_xmit(struct sk_buff *skb)
+{
+	struct llc_sap_state_ev *ev = llc_sap_ev(skb);
+	struct sk_buff *nskb;
+	int rc;
+
+	rc = llc_mac_hdr_init(skb, ev->saddr.mac, ev->daddr.mac);
+	if (rc)
+		return rc;
+
+	nskb = skb_clone(skb, GFP_ATOMIC);
+	if (!nskb)
+		return -ENOMEM;
+
+	if (skb->sk)
+		skb_set_owner_w(nskb, skb->sk);
+
+	return dev_queue_xmit(nskb);
+}
+
 /**
  *	llc_sap_action_send_ui - sends UI PDU resp to UNITDATA REQ to MAC layer
  *	@sap: SAP
···
 int llc_sap_action_send_ui(struct llc_sap *sap, struct sk_buff *skb)
 {
 	struct llc_sap_state_ev *ev = llc_sap_ev(skb);
-	int rc;
 
 	llc_pdu_header_init(skb, LLC_PDU_TYPE_U, ev->saddr.lsap,
 			    ev->daddr.lsap, LLC_PDU_CMD);
 	llc_pdu_init_as_ui_cmd(skb);
-	rc = llc_mac_hdr_init(skb, ev->saddr.mac, ev->daddr.mac);
-	if (likely(!rc)) {
-		skb_get(skb);
-		rc = dev_queue_xmit(skb);
-	}
-	return rc;
+
+	return llc_prepare_and_xmit(skb);
 }
 
 /**
···
 int llc_sap_action_send_xid_c(struct llc_sap *sap, struct sk_buff *skb)
 {
 	struct llc_sap_state_ev *ev = llc_sap_ev(skb);
-	int rc;
 
 	llc_pdu_header_init(skb, LLC_PDU_TYPE_U_XID, ev->saddr.lsap,
 			    ev->daddr.lsap, LLC_PDU_CMD);
 	llc_pdu_init_as_xid_cmd(skb, LLC_XID_NULL_CLASS_2, 0);
-	rc = llc_mac_hdr_init(skb, ev->saddr.mac, ev->daddr.mac);
-	if (likely(!rc)) {
-		skb_get(skb);
-		rc = dev_queue_xmit(skb);
-	}
-	return rc;
+
+	return llc_prepare_and_xmit(skb);
 }
 
 /**
···
 int llc_sap_action_send_test_c(struct llc_sap *sap, struct sk_buff *skb)
 {
 	struct llc_sap_state_ev *ev = llc_sap_ev(skb);
-	int rc;
 
 	llc_pdu_header_init(skb, LLC_PDU_TYPE_U, ev->saddr.lsap,
 			    ev->daddr.lsap, LLC_PDU_CMD);
 	llc_pdu_init_as_test_cmd(skb);
-	rc = llc_mac_hdr_init(skb, ev->saddr.mac, ev->daddr.mac);
-	if (likely(!rc)) {
-		skb_get(skb);
-		rc = dev_queue_xmit(skb);
-	}
-	return rc;
+
+	return llc_prepare_and_xmit(skb);
 }
 
 int llc_sap_action_send_test_r(struct llc_sap *sap, struct sk_buff *skb)