iwlwifi: fix skb truesize underestimation

By default, iwlwifi uses order-1 pages (8 KB) to store incoming frames,
but doesnt say so in skb->truesize.

This makes very possible to exhaust kernel memory since these skb evade
normal socket memory accounting.

As struct ieee80211_hdr is going to be pulled before calling IP stack,
there is no need to use dev_alloc_skb() to reserve NET_SKB_PAD bytes.
alloc_skb() is ok in this driver, allowing more tailroom.

Pull beginning of frame in skb header, in the hope we can reuse order-1
pages in the driver immediately for small frames and reduce their
truesize to the minimum (linear skbs)

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Cc: "John W. Linville" <linville@tuxdriver.com>
Cc: Neal Cardwell <ncardwell@google.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/iwlwifi/iwl-agn-rx.c

@@ -773,8 +773,7 @@ static void iwlagn_pass_packet_to_mac80211(struct iwl_priv *priv,
 	struct sk_buff *skb;
 	__le16 fc = hdr->frame_control;
 	struct iwl_rxon_context *ctx;
-	struct page *p;
-	int offset;
+	unsigned int hdrlen, fraglen;
 
 	/* We only process data packets if the interface is open */
 	if (unlikely(!priv->is_open)) {
@@ -788,16 +787,24 @@ static void iwlagn_pass_packet_to_mac80211(struct iwl_priv *priv,
 	    iwlagn_set_decrypted_flag(priv, hdr, ampdu_status, stats))
 		return;
 
-	skb = dev_alloc_skb(128);
+	/* Dont use dev_alloc_skb(), we'll have enough headroom once
+	 * ieee80211_hdr pulled.
+	 */
+	skb = alloc_skb(128, GFP_ATOMIC);
 	if (!skb) {
-		IWL_ERR(priv, "dev_alloc_skb failed\n");
+		IWL_ERR(priv, "alloc_skb failed\n");
 		return;
 	}
+	hdrlen = min_t(unsigned int, len, skb_tailroom(skb));
+	memcpy(skb_put(skb, hdrlen), hdr, hdrlen);
+	fraglen = len - hdrlen;
 
-	offset = (void *)hdr - rxb_addr(rxb);
-	p = rxb_steal_page(rxb);
-	skb_add_rx_frag(skb, 0, p, offset, len, len);
+	if (fraglen) {
+		int offset = (void *)hdr + hdrlen - rxb_addr(rxb);
 
+		skb_add_rx_frag(skb, 0, rxb_steal_page(rxb), offset,
+				fraglen, rxb->truesize);
+	}
 	iwl_update_stats(priv, false, fc, len);
 
 	/*

drivers/net/wireless/iwlwifi/iwl-trans-pcie-rx.c

@@ -374,8 +374,9 @@ static void iwl_rx_handle_rxbuf(struct iwl_trans *trans,
 	if (WARN_ON(!rxb))
 		return;
 
+	rxcb.truesize = PAGE_SIZE << hw_params(trans).rx_page_order;
 	dma_unmap_page(trans->dev, rxb->page_dma,
-		       PAGE_SIZE << hw_params(trans).rx_page_order,
+		       rxcb.truesize,
 		       DMA_FROM_DEVICE);
 
 	rxcb._page = rxb->page;

drivers/net/wireless/iwlwifi/iwl-trans.h

@@ -260,6 +260,7 @@ static inline void iwl_free_resp(struct iwl_host_cmd *cmd)
 
 struct iwl_rx_cmd_buffer {
 	struct page *_page;
+	unsigned int truesize;
 };
 
 static inline void *rxb_addr(struct iwl_rx_cmd_buffer *r)