CVE-2021-47540 mt76: mt7915: fix NULL pointer dereference in mt7915_get_phy_mode

In the Linux kernel, the following vulnerability has been resolved:

mt76: mt7915: fix NULL pointer dereference in mt7915_get_phy_mode

Fix the following NULL pointer dereference in mt7915_get_phy_mode
routine adding an ibss interface to the mt7915 driver.

[ 101.137097] wlan0: Trigger new scan to find an IBSS to join
[ 102.827039] wlan0: Creating new IBSS network, BSSID 26:a4:50:1a:6e:69
[ 103.064756] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
[ 103.073670] Mem abort info:
[ 103.076520] ESR = 0x96000005
[ 103.079614] EC = 0x25: DABT (current EL), IL = 32 bits
[ 103.084934] SET = 0, FnV = 0
[ 103.088042] EA = 0, S1PTW = 0
[ 103.091215] Data abort info:
[ 103.094104] ISV = 0, ISS = 0x00000005
[ 103.098041] CM = 0, WnR = 0
[ 103.101044] user pgtable: 4k pages, 39-bit VAs, pgdp=00000000460b1000
[ 103.107565] [0000000000000000] pgd=0000000000000000, p4d=0000000000000000, pud=0000000000000000
[ 103.116590] Internal error: Oops: 96000005 [#1] SMP
[ 103.189066] CPU: 1 PID: 333 Comm: kworker/u4:3 Not tainted 5.10.75 #0
[ 103.195498] Hardware name: MediaTek MT7622 RFB1 board (DT)
[ 103.201124] Workqueue: phy0 ieee80211_iface_work [mac80211]
[ 103.206695] pstate: 20000005 (nzCv daif -PAN -UAO -TCO BTYPE=–)
[ 103.212705] pc : mt7915_get_phy_mode+0x68/0x120 [mt7915e]
[ 103.218103] lr : mt7915_mcu_add_bss_info+0x11c/0x760 [mt7915e]
[ 103.223927] sp : ffffffc011cdb9e0
[ 103.227235] x29: ffffffc011cdb9e0 x28: ffffff8006563098
[ 103.232545] x27: ffffff8005f4da22 x26: ffffff800685ac40
[ 103.237855] x25: 0000000000000001 x24: 000000000000011f
[ 103.243165] x23: ffffff8005f4e260 x22: ffffff8006567918
[ 103.248475] x21: ffffff8005f4df80 x20: ffffff800685ac58
[ 103.253785] x19: ffffff8006744400 x18: 0000000000000000
[ 103.259094] x17: 0000000000000000 x16: 0000000000000001
[ 103.264403] x15: 000899c3a2d9d2e4 x14: 000899bdc3c3a1c8
[ 103.269713] x13: 0000000000000000 x12: 0000000000000000
[ 103.275024] x11: ffffffc010e30c20 x10: 0000000000000000
[ 103.280333] x9 : 0000000000000050 x8 : ffffff8006567d88
[ 103.285642] x7 : ffffff8006563b5c x6 : ffffff8006563b44
[ 103.290952] x5 : 0000000000000002 x4 : 0000000000000001
[ 103.296262] x3 : 0000000000000001 x2 : 0000000000000001
[ 103.301572] x1 : 0000000000000000 x0 : 0000000000000011
[ 103.306882] Call trace:
[ 103.309328] mt7915_get_phy_mode+0x68/0x120 [mt7915e]
[ 103.314378] mt7915_bss_info_changed+0x198/0x200 [mt7915e]
[ 103.319941] ieee80211_bss_info_change_notify+0x128/0x290 [mac80211]
[ 103.326360] __ieee80211_sta_join_ibss+0x308/0x6c4 [mac80211]
[ 103.332171] ieee80211_sta_create_ibss+0x8c/0x10c [mac80211]
[ 103.337895] ieee80211_ibss_work+0x3dc/0x614 [mac80211]
[ 103.343185] ieee80211_iface_work+0x388/0x3f0 [mac80211]
[ 103.348495] process_one_work+0x288/0x690
[ 103.352499] worker_thread+0x70/0x464
[ 103.356157] kthread+0x144/0x150
[ 103.359380] ret_from_fork+0x10/0x18
[ 103.362952] Code: 394008c3 52800220 394000e4 7100007f (39400023)