[PATCH v3 5/5] staging: rtl8723bs: fix negative length in WEP decryption

[Delene Tchio Romuald <delenetchior1@gmail.com>]

In rtw_wep_decrypt(), length is declared as signed int and computed as:

  length = len - hdrlen - iv_len;

If the received frame is shorter than the combined header and IV
lengths, length becomes negative. It is then passed to arc4_crypt()
which takes a u32 parameter, causing the negative value to be
implicitly cast to a very large unsigned value (e.g., -8 becomes
4294967288). This results in a massive out-of-bounds read and write
on the heap via arc4_crypt(), and a similar overflow at the
subsequent crc32_le() call using length - 4.

Add a minimum frame length check before the subtraction to ensure
length is always positive.

Found by reviewing memory operations in the driver.
Not tested on hardware.

Signed-off-by: Delene Tchio Romuald <delenetchior1@gmail.com>
---
v3:
 - Rebased on staging-next
 - Sent as numbered series with proper Cc from get_maintainer.pl

 drivers/staging/rtl8723bs/core/rtw_security.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/staging/rtl8723bs/core/rtw_security.c b/drivers/staging/rtl8723bs/core/rtw_security.c
index a00504ff29109..f3bc2240749a4 100644
--- a/drivers/staging/rtl8723bs/core/rtw_security.c
+++ b/drivers/staging/rtl8723bs/core/rtw_security.c
@@ -113,6 +113,12 @@ void rtw_wep_decrypt(struct adapter  *padapter, u8 *precvframe)
 		memcpy(&wepkey[0], iv, 3);
 		/* memcpy(&wepkey[3], &psecuritypriv->dot11DefKey[psecuritypriv->dot11PrivacyKeyIndex].skey[0], keylength); */
 		memcpy(&wepkey[3], &psecuritypriv->dot11DefKey[keyindex].skey[0], keylength);
+
+		/* Ensure the frame is long enough for WEP decryption */
+		if (((union recv_frame *)precvframe)->u.hdr.len <=
+		    prxattrib->hdrlen + prxattrib->iv_len)
+			return;
+
 		length = ((union recv_frame *)precvframe)->u.hdr.len - prxattrib->hdrlen - prxattrib->iv_len;
 
 		payload = pframe + prxattrib->iv_len + prxattrib->hdrlen;
-- 
2.43.0