wifi: rtlwifi: Fix global-out-of-bounds bug in _rtl8812ae_phy_set_txpower_limit()

There is a global-out-of-bounds reported by KASAN:

  BUG: KASAN: global-out-of-bounds in
  _rtl8812ae_eq_n_byte.part.0+0x3d/0x84 [rtl8821ae]
  Read of size 1 at addr ffffffffa0773c43 by task NetworkManager/411

  CPU: 6 PID: 411 Comm: NetworkManager Tainted: G      D
  6.1.0-rc8+ #144 e15588508517267d37
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009),
  Call Trace:
   <TASK>
   ...
   kasan_report+0xbb/0x1c0
   _rtl8812ae_eq_n_byte.part.0+0x3d/0x84 [rtl8821ae]
   rtl8821ae_phy_bb_config.cold+0x346/0x641 [rtl8821ae]
   rtl8821ae_hw_init+0x1f5e/0x79b0 [rtl8821ae]
   ...
   </TASK>

The root cause of the problem is that the comparison order of
"prate_section" in _rtl8812ae_phy_set_txpower_limit() is wrong. The
_rtl8812ae_eq_n_byte() is used to compare the first n bytes of the two
strings from tail to head, which causes the problem. In the
_rtl8812ae_phy_set_txpower_limit(), it was originally intended to meet
this requirement by carefully designing the comparison order.
For example, "pregulation" and "pbandwidth" are compared in order of
length from small to large, first is 3 and last is 4. However, the
comparison order of "prate_section" dose not obey such order requirement,
therefore when "prate_section" is "HT", when comparing from tail to head,
it will lead to access out of bounds in _rtl8812ae_eq_n_byte(). As
mentioned above, the _rtl8812ae_eq_n_byte() has the same function as
strcmp(), so just strcmp() is enough.

Fix it by removing _rtl8812ae_eq_n_byte() and use strcmp() barely.
Although it can be fixed by adjusting the comparison order of
"prate_section", this may cause the value of "rate_section" to not be
from 0 to 5. In addition, commit "21e4b072" not only moved driver
from staging to regular tree, but also added setting txpower limit
function during the driver config phase, so the problem was introduced
by this commit.

Fixes: 21e4b072 ("rtlwifi: rtl8821ae: Move driver from staging to regular tree")
Signed-off-by: Li Zetao <lizetao1@huawei.com>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20221212025812.1541311-1-lizetao1@huawei.com

drivers/net/wireless/realtek/rtlwifi/rtl8821ae/phy.c

@@ -1598,18 +1598,6 @@ static bool _rtl8812ae_get_integer_from_string(const char *str, u8 *pint)
 	return true;
 }
 
-static bool _rtl8812ae_eq_n_byte(const char *str1, const char *str2, u32 num)
-{
-	if (num == 0)
-		return false;
-	while (num > 0) {
-		num--;
-		if (str1[num] != str2[num])
-			return false;
-	}
-	return true;
-}
-
 static s8 _rtl8812ae_phy_get_chnl_idx_of_txpwr_lmt(struct ieee80211_hw *hw,
 					      u8 band, u8 channel)
 {
@@ -1659,42 +1647,42 @@ static void _rtl8812ae_phy_set_txpower_limit(struct ieee80211_hw *hw,
 	power_limit = power_limit > MAX_POWER_INDEX ?
 		      MAX_POWER_INDEX : power_limit;
 
-	if (_rtl8812ae_eq_n_byte(pregulation, "FCC", 3))
+	if (strcmp(pregulation, "FCC") == 0)
 		regulation = 0;
-	else if (_rtl8812ae_eq_n_byte(pregulation, "MKK", 3))
+	else if (strcmp(pregulation, "MKK") == 0)
 		regulation = 1;
-	else if (_rtl8812ae_eq_n_byte(pregulation, "ETSI", 4))
+	else if (strcmp(pregulation, "ETSI") == 0)
 		regulation = 2;
-	else if (_rtl8812ae_eq_n_byte(pregulation, "WW13", 4))
+	else if (strcmp(pregulation, "WW13") == 0)
 		regulation = 3;
 
-	if (_rtl8812ae_eq_n_byte(prate_section, "CCK", 3))
+	if (strcmp(prate_section, "CCK") == 0)
 		rate_section = 0;
-	else if (_rtl8812ae_eq_n_byte(prate_section, "OFDM", 4))
+	else if (strcmp(prate_section, "OFDM") == 0)
 		rate_section = 1;
-	else if (_rtl8812ae_eq_n_byte(prate_section, "HT", 2) &&
-		 _rtl8812ae_eq_n_byte(prf_path, "1T", 2))
+	else if (strcmp(prate_section, "HT") == 0 &&
+		 strcmp(prf_path, "1T") == 0)
 		rate_section = 2;
-	else if (_rtl8812ae_eq_n_byte(prate_section, "HT", 2) &&
-		 _rtl8812ae_eq_n_byte(prf_path, "2T", 2))
+	else if (strcmp(prate_section, "HT") == 0 &&
+		 strcmp(prf_path, "2T") == 0)
 		rate_section = 3;
-	else if (_rtl8812ae_eq_n_byte(prate_section, "VHT", 3) &&
-		 _rtl8812ae_eq_n_byte(prf_path, "1T", 2))
+	else if (strcmp(prate_section, "VHT") == 0 &&
+		 strcmp(prf_path, "1T") == 0)
 		rate_section = 4;
-	else if (_rtl8812ae_eq_n_byte(prate_section, "VHT", 3) &&
-		 _rtl8812ae_eq_n_byte(prf_path, "2T", 2))
+	else if (strcmp(prate_section, "VHT") == 0 &&
+		 strcmp(prf_path, "2T") == 0)
 		rate_section = 5;
 
-	if (_rtl8812ae_eq_n_byte(pbandwidth, "20M", 3))
+	if (strcmp(pbandwidth, "20M") == 0)
 		bandwidth = 0;
-	else if (_rtl8812ae_eq_n_byte(pbandwidth, "40M", 3))
+	else if (strcmp(pbandwidth, "40M") == 0)
 		bandwidth = 1;
-	else if (_rtl8812ae_eq_n_byte(pbandwidth, "80M", 3))
+	else if (strcmp(pbandwidth, "80M") == 0)
 		bandwidth = 2;
-	else if (_rtl8812ae_eq_n_byte(pbandwidth, "160M", 4))
+	else if (strcmp(pbandwidth, "160M") == 0)
 		bandwidth = 3;
 
-	if (_rtl8812ae_eq_n_byte(pband, "2.4G", 4)) {
+	if (strcmp(pband, "2.4G") == 0) {
 		ret = _rtl8812ae_phy_get_chnl_idx_of_txpwr_lmt(hw,
 							       BAND_ON_2_4G,
 							       channel);
@@ -1718,7 +1706,7 @@ static void _rtl8812ae_phy_set_txpower_limit(struct ieee80211_hw *hw,
 			regulation, bandwidth, rate_section, channel_index,
 			rtlphy->txpwr_limit_2_4g[regulation][bandwidth]
 				[rate_section][channel_index][RF90_PATH_A]);
-	} else if (_rtl8812ae_eq_n_byte(pband, "5G", 2)) {
+	} else if (strcmp(pband, "5G") == 0) {
 		ret = _rtl8812ae_phy_get_chnl_idx_of_txpwr_lmt(hw,
 							       BAND_ON_5G,
 							       channel);