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Subject[PATCH] [34/139] mac80211: Fix BUG in pskb_expand_head when transmitting shared skbs
Date
2.6.35-longterm review patch.  If anyone has any objections, please let me know.

------------------
From: Helmut Schaa <helmut.schaa@googlemail.com>

commit 7e2447075690860e2cea96b119fc9cadbaa7e83c upstream.

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>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Andi Kleen <ak@linux.intel.com>

---
net/mac80211/tx.c | 21 ++++++++++++++++++---
1 file changed, 18 insertions(+), 3 deletions(-)

Index: linux-2.6.35.y/net/mac80211/tx.c
===================================================================
--- linux-2.6.35.y.orig/net/mac80211/tx.c
+++ linux-2.6.35.y/net/mac80211/tx.c
@@ -1679,15 +1679,13 @@ netdev_tx_t ieee80211_subif_start_xmit(s
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];
@@ -1859,6 +1857,20 @@ netdev_tx_t ieee80211_subif_start_xmit(s
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;
@@ -1877,6 +1889,9 @@ netdev_tx_t ieee80211_subif_start_xmit(s
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;

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