mac80211: Fix BUG in pskb_expand_head when transmitting shared skbs

mac80211 doesn't handle shared skbs correctly at the moment. As a result
a possible resize can trigger a BUG in pskb_expand_head.

[  676.030000] Kernel bug detected[#1]:
[  676.030000] Cpu 0
[  676.030000] $ 0   : 00000000 00000000 819662ff 00000002
[  676.030000] $ 4   : 81966200 00000020 00000000 00000020
[  676.030000] $ 8   : 819662e0 800043c0 00000002 00020000
[  676.030000] $12   : 3b9aca00 00000000 00000000 00470000
[  676.030000] $16   : 80ea2000 00000000 00000000 00000000
[  676.030000] $20   : 818aa200 80ea2018 80ea2000 00000008
[  676.030000] $24   : 00000002 800ace5c
[  676.030000] $28   : 8199a000 8199bd20 81938f88 80f180d4
[  676.030000] Hi    : 0000026e
[  676.030000] Lo    : 0000757e
[  676.030000] epc   : 801245e4 pskb_expand_head+0x44/0x1d8
[  676.030000]     Not tainted
[  676.030000] ra    : 80f180d4 ieee80211_skb_resize+0xb0/0x114 [mac80211]
[  676.030000] Status: 1000a403    KERNEL EXL IE
[  676.030000] Cause : 10800024
[  676.030000] PrId  : 0001964c (MIPS 24Kc)
[  676.030000] Modules linked in: mac80211_hwsim rt2800lib rt2x00soc rt2x00pci rt2x00lib mac80211 crc_itu_t crc_ccitt cfg80211 compat arc4 aes_generic deflate ecb cbc [last unloaded: rt2800pci]
[  676.030000] Process kpktgend_0 (pid: 97, threadinfo=8199a000, task=81879f48, tls=00000000)
[  676.030000] Stack : ffffffff 00000000 00000000 00000014 00000004 80ea2000 00000000 00000000
[  676.030000]         818aa200 80f180d4 ffffffff 0000000a 81879f78 81879f48 81879f48 00000018
[  676.030000]         81966246 80ea2000 818432e0 80f1a420 80203050 81814d98 00000001 81879f48
[  676.030000]         81879f48 00000018 81966246 818432e0 0000001a 8199bdd4 0000001c 80f1b72c
[  676.030000]         80203020 8001292c 80ef4aa2 7f10b55d 801ab5b8 81879f48 00000188 80005c90
[  676.030000]         ...
[  676.030000] Call Trace:
[  676.030000] [<801245e4>] pskb_expand_head+0x44/0x1d8
[  676.030000] [<80f180d4>] ieee80211_skb_resize+0xb0/0x114 [mac80211]
[  676.030000] [<80f1a420>] ieee80211_xmit+0x150/0x22c [mac80211]
[  676.030000] [<80f1b72c>] ieee80211_subif_start_xmit+0x6f4/0x73c [mac80211]
[  676.030000] [<8014361c>] pktgen_thread_worker+0xfac/0x16f8
[  676.030000] [<8002ebe8>] kthread+0x7c/0x88
[  676.030000] [<80008e0c>] kernel_thread_helper+0x10/0x18
[  676.030000]
[  676.030000]
[  676.030000] Code: 24020001  10620005  2502001f <0200000d> 0804917a  00000000  2502001f  00441023  00531021

Fix this by making a local copy of shared skbs prior to mangeling them.
To avoid copying the skb unnecessarily move the skb_copy call below the
checks that don't need write access to the skb.

Also, move the assignment of nh_pos and h_pos below the skb_copy to point
to the correct skb.

It would be possible to avoid another resize of the copied skb by using
skb_copy_expand instead of skb_copy but that would make the patch more
complex. Also, shared skbs are a corner case right now, so the resize
shouldn't matter much.

Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/tx.c

@@ -1737,15 +1737,13 @@ netdev_tx_t ieee80211_subif_start_xmit(struct sk_buff *skb,
 	int nh_pos, h_pos;
 	struct sta_info *sta = NULL;
 	u32 sta_flags = 0;
+	struct sk_buff *tmp_skb;
 
 	if (unlikely(skb->len < ETH_HLEN)) {
 		ret = NETDEV_TX_OK;
 		goto fail;
 	}
 
-	nh_pos = skb_network_header(skb) - skb->data;
-	h_pos = skb_transport_header(skb) - skb->data;
-
 	/* convert Ethernet header to proper 802.11 header (based on
 	 * operation mode) */
 	ethertype = (skb->data[12] << 8) | skb->data[13];
@@ -1918,6 +1916,20 @@ netdev_tx_t ieee80211_subif_start_xmit(struct sk_buff *skb,
 		goto fail;
 	}
 
+	/*
+	 * If the skb is shared we need to obtain our own copy.
+	 */
+	if (skb_shared(skb)) {
+		tmp_skb = skb;
+		skb = skb_copy(skb, GFP_ATOMIC);
+		kfree_skb(tmp_skb);
+
+		if (!skb) {
+			ret = NETDEV_TX_OK;
+			goto fail;
+		}
+	}
+
 	hdr.frame_control = fc;
 	hdr.duration_id = 0;
 	hdr.seq_ctrl = 0;
@@ -1936,6 +1948,9 @@ netdev_tx_t ieee80211_subif_start_xmit(struct sk_buff *skb,
 		encaps_len = 0;
 	}
 
+	nh_pos = skb_network_header(skb) - skb->data;
+	h_pos = skb_transport_header(skb) - skb->data;
+
 	skb_pull(skb, skip_header_bytes);
 	nh_pos -= skip_header_bytes;
 	h_pos -= skip_header_bytes;