[PATCH] staging: r8188eu: fix a potential integer underflow bug

[Dan Carpenter <error27@gmail.com>]

Here the code is testing to see if skb->len meets a minimum size
requirement.  However if skb->len is very small then the ETH_HLEN
subtraction will result in a negative which is then type promoted
to an unsigned int and the condition will be true.

Generally, when you have an untrusted variable like skb->len, you
should move all the math to the other side of the comparison.

Fixes: 15865124feed ("staging: r8188eu: introduce new core dir for RTL8188eu driver")
Signed-off-by: Dan Carpenter <error27@gmail.com>
---
Compile tested only.  This is basic algebra of moving parts of the
equation from one side to the other and I am surprisingly bad at
something that I was supposed to have learned in 9th grade.

 drivers/staging/r8188eu/core/rtw_br_ext.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/staging/r8188eu/core/rtw_br_ext.c b/drivers/staging/r8188eu/core/rtw_br_ext.c
index a7c67014dde0..f49e32c33372 100644
--- a/drivers/staging/r8188eu/core/rtw_br_ext.c
+++ b/drivers/staging/r8188eu/core/rtw_br_ext.c
@@ -538,7 +538,7 @@ int nat25_db_handle(struct adapter *priv, struct sk_buff *skb, int method)
 		/*------------------------------------------------*/
 		struct ipv6hdr *iph = (struct ipv6hdr *)(skb->data + ETH_HLEN);
 
-		if (sizeof(*iph) >= (skb->len - ETH_HLEN))
+		if (skb->len <= sizeof(*iph) + ETH_HLEN)
 			return -1;
 
 		switch (method) {
-- 
2.39.1