mac80211: fix AP powersave TX vs. wakeup race

There is a race between the TX path and the STA wakeup: while
a station is sleeping, mac80211 buffers frames until it wakes
up, then the frames are transmitted. However, the RX and TX
path are concurrent, so the packet indicating wakeup can be
processed while a packet is being transmitted.

This can lead to a situation where the buffered frames list
is emptied on the one side, while a frame is being added on
the other side, as the station is still seen as sleeping in
the TX path.

As a result, the newly added frame will not be send anytime
soon. It might be sent much later (and out of order) when the
station goes to sleep and wakes up the next time.

Additionally, it can lead to the crash below.

Fix all this by synchronising both paths with a new lock.
Both path are not fastpath since they handle PS situations.

In a later patch we'll remove the extra skb queue locks to
reduce locking overhead.

BUG: unable to handle kernel
NULL pointer dereference at 000000b0
IP: [<ff6f1791>] ieee80211_report_used_skb+0x11/0x3e0 [mac80211]
*pde = 00000000
Oops: 0000 [#1] SMP DEBUG_PAGEALLOC
EIP: 0060:[<ff6f1791>] EFLAGS: 00210282 CPU: 1
EIP is at ieee80211_report_used_skb+0x11/0x3e0 [mac80211]
EAX: e5900da0 EBX: 00000000 ECX: 00000001 EDX: 00000000
ESI: e41d00c0 EDI: e5900da0 EBP: ebe458e4 ESP: ebe458b0
 DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068
CR0: 8005003b CR2: 000000b0 CR3: 25a78000 CR4: 000407d0
DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3: 00000000
DR6: ffff0ff0 DR7: 00000400
Process iperf (pid: 3934, ti=ebe44000 task=e757c0b0 task.ti=ebe44000)
iwlwifi 0000:02:00.0: I iwl_pcie_enqueue_hcmd Sending command LQ_CMD (#4e), seq: 0x0903, 92 bytes at 3[3]:9
Stack:
 e403b32c ebe458c4 00200002 00200286 e403b338 ebe458cc c10960bb e5900da0
 ff76a6ec ebe458d8 00000000 e41d00c0 e5900da0 ebe458f0 ff6f1b75 e403b210
 ebe4598c ff723dc1 00000000 ff76a6ec e597c978 e403b758 00000002 00000002
Call Trace:
 [<ff6f1b75>] ieee80211_free_txskb+0x15/0x20 [mac80211]
 [<ff723dc1>] invoke_tx_handlers+0x1661/0x1780 [mac80211]
 [<ff7248a5>] ieee80211_tx+0x75/0x100 [mac80211]
 [<ff7249bf>] ieee80211_xmit+0x8f/0xc0 [mac80211]
 [<ff72550e>] ieee80211_subif_start_xmit+0x4fe/0xe20 [mac80211]
 [<c149ef70>] dev_hard_start_xmit+0x450/0x950
 [<c14b9aa9>] sch_direct_xmit+0xa9/0x250
 [<c14b9c9b>] __qdisc_run+0x4b/0x150
 [<c149f732>] dev_queue_xmit+0x2c2/0xca0

Cc: stable@vger.kernel.org
Reported-by: Yaara Rozenblum <yaara.rozenblum@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Reviewed-by: Stanislaw Gruszka <sgruszka@redhat.com>
[reword commit log, use a separate lock]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
ieee80211_sta_ps_deliver_response(struct sta_info *sta,
				  int n_frames, u8 ignored_acs,
				  enum ieee80211_frame_release_type reason)
{
	struct ieee80211_sub_if_data *sdata = sta->sdata;
	struct ieee80211_local *local = sdata->local;
	bool more_data = false;
	int ac;
	unsigned long driver_release_tids = 0;
	struct sk_buff_head frames;

	/* Service or PS-Poll period starts */
	set_sta_flag(sta, WLAN_STA_SP);

	__skb_queue_head_init(&frames);

	/* Get response frame(s) and more data bit for the last one. */
	for (ac = 0; ac < IEEE80211_NUM_ACS; ac++) {
		unsigned long tids;

		if (ignored_acs & BIT(ac))
			continue;

		tids = ieee80211_tids_for_ac(ac);

		/* if we already have frames from software, then we can't also
		 * release from hardware queues
		 */
		if (skb_queue_empty(&frames))
			driver_release_tids |= sta->driver_buffered_tids & tids;

		if (driver_release_tids) {
			/* If the driver has data on more than one TID then
			 * certainly there's more data if we release just a
			 * single frame now (from a single TID). This will
			 * only happen for PS-Poll.
			 */
			if (reason == IEEE80211_FRAME_RELEASE_PSPOLL &&
			    hweight16(driver_release_tids) > 1) {
				more_data = true;
				driver_release_tids =
					BIT(find_highest_prio_tid(
						driver_release_tids));
				break;
			}
		} else {
			struct sk_buff *skb;

			while (n_frames > 0) {
				skb = skb_dequeue(&sta->tx_filtered[ac]);
				if (!skb) {
					skb = skb_dequeue(
						&sta->ps_tx_buf[ac]);
					if (skb)
						local->total_ps_buffered--;
				}
				if (!skb)
					break;
				n_frames--;
				__skb_queue_tail(&frames, skb);
			}
		}

		/* If we have more frames buffered on this AC, then set the
		 * more-data bit and abort the loop since we can't send more
		 * data from other ACs before the buffered frames from this.
		 */
		if (!skb_queue_empty(&sta->tx_filtered[ac]) ||
		    !skb_queue_empty(&sta->ps_tx_buf[ac])) {
			more_data = true;
			break;
		}
	}

	if (skb_queue_empty(&frames) && !driver_release_tids) {
		int tid;

		/*
		 * For PS-Poll, this can only happen due to a race condition
		 * when we set the TIM bit and the station notices it, but
		 * before it can poll for the frame we expire it.
		 *
		 * For uAPSD, this is said in the standard (11.2.1.5 h):
		 *	At each unscheduled SP for a non-AP STA, the AP shall
		 *	attempt to transmit at least one MSDU or MMPDU, but no
		 *	more than the value specified in the Max SP Length field
		 *	in the QoS Capability element from delivery-enabled ACs,
		 *	that are destined for the non-AP STA.
		 *
		 * Since we have no other MSDU/MMPDU, transmit a QoS null frame.
		 */

		/* This will evaluate to 1, 3, 5 or 7. */
		tid = 7 - ((ffs(~ignored_acs) - 1) << 1);

		ieee80211_send_null_response(sdata, sta, tid, reason, true);
	} else if (!driver_release_tids) {
		struct sk_buff_head pending;
		struct sk_buff *skb;
		int num = 0;
		u16 tids = 0;
		bool need_null = false;

		skb_queue_head_init(&pending);

		while ((skb = __skb_dequeue(&frames))) {
			struct ieee80211_tx_info *info = IEEE80211_SKB_CB(skb);
			struct ieee80211_hdr *hdr = (void *) skb->data;
			u8 *qoshdr = NULL;

			num++;

			/*
			 * Tell TX path to send this frame even though the
			 * STA may still remain is PS mode after this frame
			 * exchange.
			 */
			info->flags |= IEEE80211_TX_CTL_NO_PS_BUFFER |
				       IEEE80211_TX_CTL_PS_RESPONSE;

			/*
			 * Use MoreData flag to indicate whether there are
			 * more buffered frames for this STA
			 */
			if (more_data || !skb_queue_empty(&frames))
				hdr->frame_control |=
					cpu_to_le16(IEEE80211_FCTL_MOREDATA);
			else
				hdr->frame_control &=
					cpu_to_le16(~IEEE80211_FCTL_MOREDATA);

			if (ieee80211_is_data_qos(hdr->frame_control) ||
			    ieee80211_is_qos_nullfunc(hdr->frame_control))
				qoshdr = ieee80211_get_qos_ctl(hdr);

			tids |= BIT(skb->priority);

			__skb_queue_tail(&pending, skb);

			/* end service period after last frame or add one */
			if (!skb_queue_empty(&frames))
				continue;

			if (reason != IEEE80211_FRAME_RELEASE_UAPSD) {
				/* for PS-Poll, there's only one frame */
				info->flags |= IEEE80211_TX_STATUS_EOSP |
					       IEEE80211_TX_CTL_REQ_TX_STATUS;
				break;
			}

			/* For uAPSD, things are a bit more complicated. If the
			 * last frame has a QoS header (i.e. is a QoS-data or
			 * QoS-nulldata frame) then just set the EOSP bit there
			 * and be done.
			 * If the frame doesn't have a QoS header (which means
			 * it should be a bufferable MMPDU) then we can't set
			 * the EOSP bit in the QoS header; add a QoS-nulldata
			 * frame to the list to send it after the MMPDU.
			 *
			 * Note that this code is only in the mac80211-release
			 * code path, we assume that the driver will not buffer
			 * anything but QoS-data frames, or if it does, will
			 * create the QoS-nulldata frame by itself if needed.
			 *
			 * Cf. 802.11-2012 10.2.1.10 (c).
			 */
			if (qoshdr) {
				*qoshdr |= IEEE80211_QOS_CTL_EOSP;

				info->flags |= IEEE80211_TX_STATUS_EOSP |
					       IEEE80211_TX_CTL_REQ_TX_STATUS;
			} else {
				/* The standard isn't completely clear on this
				 * as it says the more-data bit should be set
				 * if there are more BUs. The QoS-Null frame
				 * we're about to send isn't buffered yet, we
				 * only create it below, but let's pretend it
				 * was buffered just in case some clients only
				 * expect more-data=0 when eosp=1.
				 */
				hdr->frame_control |=
					cpu_to_le16(IEEE80211_FCTL_MOREDATA);
				need_null = true;
				num++;
			}
			break;
		}

		drv_allow_buffered_frames(local, sta, tids, num,
					  reason, more_data);

		ieee80211_add_pending_skbs(local, &pending);

		if (need_null)
			ieee80211_send_null_response(
				sdata, sta, find_highest_prio_tid(tids),
				reason, false);

		sta_info_recalc_tim(sta);
	} else {
		/*
		 * We need to release a frame that is buffered somewhere in the
		 * driver ... it'll have to handle that.
		 * Note that the driver also has to check the number of frames
		 * on the TIDs we're releasing from - if there are more than
		 * n_frames it has to set the more-data bit (if we didn't ask
		 * it to set it anyway due to other buffered frames); if there
		 * are fewer than n_frames it has to make sure to adjust that
		 * to allow the service period to end properly.
		 */
		drv_release_buffered_frames(local, sta, driver_release_tids,
					    n_frames, reason, more_data);

		/*
		 * Note that we don't recalculate the TIM bit here as it would
		 * most likely have no effect at all unless the driver told us
		 * that the TID(s) became empty before returning here from the
		 * release function.
		 * Either way, however, when the driver tells us that the TID(s)
		 * became empty we'll do the TIM recalculation.
		 */
	}
}

ieee80211_sta_ps_deliver_response(struct sta_info *sta,
				  int n_frames, u8 ignored_acs,
				  enum ieee80211_frame_release_type reason)
{
	struct ieee80211_sub_if_data *sdata = sta->sdata;
	struct ieee80211_local *local = sdata->local;
	bool more_data = false;
	int ac;
	unsigned long driver_release_tids = 0;
	struct sk_buff_head frames;

	/* Service or PS-Poll period starts */
	set_sta_flag(sta, WLAN_STA_SP);

	__skb_queue_head_init(&frames);

	/* Get response frame(s) and more data bit for the last one. */
	for (ac = 0; ac < IEEE80211_NUM_ACS; ac++) {
		unsigned long tids;

		if (ignored_acs & BIT(ac))
			continue;

		tids = ieee80211_tids_for_ac(ac);

		/* if we already have frames from software, then we can't also
		 * release from hardware queues
		 */
		if (skb_queue_empty(&frames))
			driver_release_tids |= sta->driver_buffered_tids & tids;

		if (driver_release_tids) {
			/* If the driver has data on more than one TID then
			 * certainly there's more data if we release just a
			 * single frame now (from a single TID). This will
			 * only happen for PS-Poll.
			 */
			if (reason == IEEE80211_FRAME_RELEASE_PSPOLL &&
			    hweight16(driver_release_tids) > 1) {
				more_data = true;
				driver_release_tids =
					BIT(find_highest_prio_tid(
						driver_release_tids));
				break;
			}
		} else {
			struct sk_buff *skb;

			while (n_frames > 0) {
				skb = skb_dequeue(&sta->tx_filtered[ac]);
				if (!skb) {
					skb = skb_dequeue(
						&sta->ps_tx_buf[ac]);
					if (skb)
						local->total_ps_buffered--;
				}
				if (!skb)
					break;
				n_frames--;
				__skb_queue_tail(&frames, skb);
			}
		}

		/* If we have more frames buffered on this AC, then set the
		 * more-data bit and abort the loop since we can't send more
		 * data from other ACs before the buffered frames from this.
		 */
		if (!skb_queue_empty(&sta->tx_filtered[ac]) ||
		    !skb_queue_empty(&sta->ps_tx_buf[ac])) {
			more_data = true;
			break;
		}
	}

	if (skb_queue_empty(&frames) && !driver_release_tids) {
		int tid;

		/*
		 * For PS-Poll, this can only happen due to a race condition
		 * when we set the TIM bit and the station notices it, but
		 * before it can poll for the frame we expire it.
		 *
		 * For uAPSD, this is said in the standard (11.2.1.5 h):
		 *	At each unscheduled SP for a non-AP STA, the AP shall
		 *	attempt to transmit at least one MSDU or MMPDU, but no
		 *	more than the value specified in the Max SP Length field
		 *	in the QoS Capability element from delivery-enabled ACs,
		 *	that are destined for the non-AP STA.
		 *
		 * Since we have no other MSDU/MMPDU, transmit a QoS null frame.
		 */

		/* This will evaluate to 1, 3, 5 or 7. */
		tid = 7 - ((ffs(~ignored_acs) - 1) << 1);

		ieee80211_send_null_response(sdata, sta, tid, reason, true);
	} else if (!driver_release_tids) {
		struct sk_buff_head pending;
		struct sk_buff *skb;
		int num = 0;
		u16 tids = 0;
		bool need_null = false;

		skb_queue_head_init(&pending);

		while ((skb = __skb_dequeue(&frames))) {
			struct ieee80211_tx_info *info = IEEE80211_SKB_CB(skb);
			struct ieee80211_hdr *hdr = (void *) skb->data;
			u8 *qoshdr = NULL;

			num++;

			/*
			 * Tell TX path to send this frame even though the
			 * STA may still remain is PS mode after this frame
			 * exchange.
			 */
			info->flags |= IEEE80211_TX_CTL_NO_PS_BUFFER |
				       IEEE80211_TX_CTL_PS_RESPONSE;

			/*
			 * Use MoreData flag to indicate whether there are
			 * more buffered frames for this STA
			 */
			if (more_data || !skb_queue_empty(&frames))
				hdr->frame_control |=
					cpu_to_le16(IEEE80211_FCTL_MOREDATA);
			else
				hdr->frame_control &=
					cpu_to_le16(~IEEE80211_FCTL_MOREDATA);

			if (ieee80211_is_data_qos(hdr->frame_control) ||
			    ieee80211_is_qos_nullfunc(hdr->frame_control))
				qoshdr = ieee80211_get_qos_ctl(hdr);

			tids |= BIT(skb->priority);

			__skb_queue_tail(&pending, skb);

			/* end service period after last frame or add one */
			if (!skb_queue_empty(&frames))
				continue;

			if (reason != IEEE80211_FRAME_RELEASE_UAPSD) {
				/* for PS-Poll, there's only one frame */
				info->flags |= IEEE80211_TX_STATUS_EOSP |
					       IEEE80211_TX_CTL_REQ_TX_STATUS;
				break;
			}

			/* For uAPSD, things are a bit more complicated. If the
			 * last frame has a QoS header (i.e. is a QoS-data or
			 * QoS-nulldata frame) then just set the EOSP bit there
			 * and be done.
			 * If the frame doesn't have a QoS header (which means
			 * it should be a bufferable MMPDU) then we can't set
			 * the EOSP bit in the QoS header; add a QoS-nulldata
			 * frame to the list to send it after the MMPDU.
			 *
			 * Note that this code is only in the mac80211-release
			 * code path, we assume that the driver will not buffer
			 * anything but QoS-data frames, or if it does, will
			 * create the QoS-nulldata frame by itself if needed.
			 *
			 * Cf. 802.11-2012 10.2.1.10 (c).
			 */
			if (qoshdr) {
				*qoshdr |= IEEE80211_QOS_CTL_EOSP;

				info->flags |= IEEE80211_TX_STATUS_EOSP |
					       IEEE80211_TX_CTL_REQ_TX_STATUS;
			} else {
				/* The standard isn't completely clear on this
				 * as it says the more-data bit should be set
				 * if there are more BUs. The QoS-Null frame
				 * we're about to send isn't buffered yet, we
				 * only create it below, but let's pretend it
				 * was buffered just in case some clients only
				 * expect more-data=0 when eosp=1.
				 */
				hdr->frame_control |=
					cpu_to_le16(IEEE80211_FCTL_MOREDATA);
				need_null = true;
				num++;
			}
			break;
		}

		drv_allow_buffered_frames(local, sta, tids, num,
					  reason, more_data);

		ieee80211_add_pending_skbs(local, &pending);

		if (need_null)
			ieee80211_send_null_response(
				sdata, sta, find_highest_prio_tid(tids),
				reason, false);

		sta_info_recalc_tim(sta);
	} else {
		/*
		 * We need to release a frame that is buffered somewhere in the
		 * driver ... it'll have to handle that.
		 * Note that the driver also has to check the number of frames
		 * on the TIDs we're releasing from - if there are more than
		 * n_frames it has to set the more-data bit (if we didn't ask
		 * it to set it anyway due to other buffered frames); if there
		 * are fewer than n_frames it has to make sure to adjust that
		 * to allow the service period to end properly.
		 */
		drv_release_buffered_frames(local, sta, driver_release_tids,
					    n_frames, reason, more_data);

		/*
		 * Note that we don't recalculate the TIM bit here as it would
		 * most likely have no effect at all unless the driver told us
		 * that the TID(s) became empty before returning here from the
		 * release function.
		 * Either way, however, when the driver tells us that the TID(s)
		 * became empty we'll do the TIM recalculation.
		 */
	}
}
