[PATCH v2] staging: rtl8723bs: fix missing shared-key auth challenge length check

[Panagiotis Petrakopoulos <npetrakopoulos2003@gmail.com>]

The WEP shared-key authentication handlers use the challenge-text
element's length. This text and its length are attacker-controlled.
The handlers do not check the length against the fixed 128-byte
chg_txt buffer.

In OnAuthClient() the length from rtw_get_ie() can be up to 255 bytes.
It is used to perform memcpy() into the 128-byte pmlmeinfo->chg_txt.
A malicious AP sending a malformed WLAN_EID_CHALLENGE element can
overflow/underfill chg_txt by up to 127 bytes. This is reachable over
the air, before association, during shared-key authentication. In the
case of an overflow, the driver can write out of bounds. In the case
of an underfill, the driver can echo stale buffer memory. In OnAuth() 
a similar issue is observed. The driver compares a full 128 bytes
regardless of the element's length, reading past a shorter element.

The challenge text is defined to be exactly 128 octets, which is
already provided as the WLAN_AUTH_CHALLENGE_LEN define; require the
element to be exactly that length in both handlers.

Fixes: 554c0a3abf21 ("staging: Add rtl8723bs sdio wifi driver")
Cc: stable@vger.kernel.org
Signed-off-by: Panagiotis Petrakopoulos <npetrakopoulos2003@gmail.com>
Reviewed-by: Manuel Ebner <manuelebner@mailbox.org>
---
v2:
improved patch description for clarity. no code changes.

testing:
Compile-tested only; I do not have RTL8723BS hardware to test the
shared-key authentication path at runtime. The change only rejects
challenge elements whose length differs from the spec-mandated 128
bytes, so conforming peers are unaffected.

 drivers/staging/rtl8723bs/core/rtw_mlme_ext.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/staging/rtl8723bs/core/rtw_mlme_ext.c b/drivers/staging/rtl8723bs/core/rtw_mlme_ext.c
index a86d6f97cf02..13634d4e83d1 100644
--- a/drivers/staging/rtl8723bs/core/rtw_mlme_ext.c
+++ b/drivers/staging/rtl8723bs/core/rtw_mlme_ext.c
@@ -787,7 +787,7 @@ unsigned int OnAuth(struct adapter *padapter, union recv_frame *precv_frame)
 			p = rtw_get_ie(pframe + WLAN_HDR_A3_LEN + 4 + _AUTH_IE_OFFSET_, WLAN_EID_CHALLENGE, (int *)&ie_len,
 					len - WLAN_HDR_A3_LEN - _AUTH_IE_OFFSET_ - 4);
 
-			if (!p || ie_len <= 0) {
+			if (!p || ie_len != WLAN_AUTH_CHALLENGE_LEN) {
 				status = WLAN_STATUS_CHALLENGE_FAIL;
 				goto auth_fail;
 			}
@@ -873,7 +873,7 @@ unsigned int OnAuthClient(struct adapter *padapter, union recv_frame *precv_fram
 			p = rtw_get_ie(pframe + WLAN_HDR_A3_LEN + _AUTH_IE_OFFSET_, WLAN_EID_CHALLENGE, (int *)&len,
 				pkt_len - WLAN_HDR_A3_LEN - _AUTH_IE_OFFSET_);
 
-			if (!p)
+			if (!p || len != WLAN_AUTH_CHALLENGE_LEN)
 				goto authclnt_fail;
 
 			memcpy(pmlmeinfo->chg_txt, p + 2, len);
-- 
2.55.0