[PATCH v3] staging: rtl8723bs: fix missing shared-key auth challenge length check

[Panagiotis Petrakopoulos <npetrakopoulos2003@gmail.com>]

The WEP shared-key authentication handler uses the challenge-text
element's attacker-controlled length without checking it against the
fixed 128-byte chg_txt buffer.

In OnAuthClient() the length from rtw_get_ie() - up to 255 - is used
to perform memcpy() into the 128-byte pmlmeinfo->chg_txt, so a
malicious AP sending a malformed WLAN_EID_CHALLENGE element can
overflow/underfill chg_txt by up to 127 bytes. It 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.

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 before use.

Fixes: 554c0a3abf21 ("staging: Add rtl8723bs sdio wifi driver")
Cc: stable@vger.kernel.org
Signed-off-by: Panagiotis Petrakopoulos <npetrakopoulos2003@gmail.com>
---
v3:
rebased onto staging-linus. OnAuth() is already fixed there, so this
revision only touches OnAuthClient(). no other code changes.

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 | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/staging/rtl8723bs/core/rtw_mlme_ext.c b/drivers/staging/rtl8723bs/core/rtw_mlme_ext.c
index a443b3530fb9..a9382dc1294b 100644
--- a/drivers/staging/rtl8723bs/core/rtw_mlme_ext.c
+++ b/drivers/staging/rtl8723bs/core/rtw_mlme_ext.c
@@ -879,7 +879,7 @@ 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