mwifiex: Fix memory corruption in dump_station

The mwifiex_cfg80211_dump_station() uses static variable for iterating
over a linked list of all associated stations (when the driver is in UAP
role). This has a race condition if .dump_station is called in parallel
for multiple interfaces. This corruption can be triggered by registering
multiple SSIDs and calling, in parallel for multiple interfaces
    iw dev <iface> station dump

[16750.719775] Unable to handle kernel paging request at virtual address dead000000000110
...
[16750.899173] Call trace:
[16750.901696]  mwifiex_cfg80211_dump_station+0x94/0x100 [mwifiex]
[16750.907824]  nl80211_dump_station+0xbc/0x278 [cfg80211]
[16750.913160]  netlink_dump+0xe8/0x320
[16750.916827]  netlink_recvmsg+0x1b4/0x338
[16750.920861]  ____sys_recvmsg+0x7c/0x2b0
[16750.924801]  ___sys_recvmsg+0x70/0x98
[16750.928564]  __sys_recvmsg+0x58/0xa0
[16750.932238]  __arm64_sys_recvmsg+0x28/0x30
[16750.936453]  el0_svc_common.constprop.3+0x90/0x158
[16750.941378]  do_el0_svc+0x74/0x90
[16750.944784]  el0_sync_handler+0x12c/0x1a8
[16750.948903]  el0_sync+0x114/0x140
[16750.952312] Code: f9400003 f907f423 eb02007f 54fffd60 (b9401060)
[16750.958583] ---[ end trace c8ad181c2f4b8576 ]---

This patch drops the use of the static iterator, and instead every time
the function is called iterates to the idx-th position of the
linked-list.

It would be better to convert the code not to use linked list for
associated stations storage (since the chip has a limited number of
associated stations anyway - it could just be an array). Such a change
may be proposed in the future. In the meantime this patch can backported
into stable kernels in this simple form.

Fixes: 8baca1a3 ("mwifiex: dump station support in uap mode")
Signed-off-by: Pali Rohár <pali@kernel.org>
Acked-by: Ganapathi Bhat <ganapathi.bhat@nxp.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200515075924.13841-1-pali@kernel.org

drivers/net/wireless/marvell/mwifiex/cfg80211.c

@@ -1496,7 +1496,8 @@ mwifiex_cfg80211_dump_station(struct wiphy *wiphy, struct net_device *dev,
 			      int idx, u8 *mac, struct station_info *sinfo)
 {
 	struct mwifiex_private *priv = mwifiex_netdev_get_priv(dev);
-	static struct mwifiex_sta_node *node;
+	struct mwifiex_sta_node *node;
+	int i;
 
 	if ((GET_BSS_ROLE(priv) == MWIFIEX_BSS_ROLE_STA) &&
 	    priv->media_connected && idx == 0) {
@@ -1506,13 +1507,10 @@ mwifiex_cfg80211_dump_station(struct wiphy *wiphy, struct net_device *dev,
 		mwifiex_send_cmd(priv, HOST_CMD_APCMD_STA_LIST,
 				 HostCmd_ACT_GEN_GET, 0, NULL, true);
 
-		if (node && (&node->list == &priv->sta_list)) {
-			node = NULL;
-			return -ENOENT;
-		}
-
-		node = list_prepare_entry(node, &priv->sta_list, list);
-		list_for_each_entry_continue(node, &priv->sta_list, list) {
+		i = 0;
+		list_for_each_entry(node, &priv->sta_list, list) {
+			if (i++ != idx)
+				continue;
 			ether_addr_copy(mac, node->mac_addr);
 			return mwifiex_dump_station_info(priv, node, sinfo);
 		}