ppp, slip: Validate VJ compression slot parameters completely

Currently slhc_init() treats out-of-range values of rslots and tslots
as equivalent to 0, except that if tslots is too large it will
dereference a null pointer (CVE-2015-7799).

Add a range-check at the top of the function and make it return an
ERR_PTR() on error instead of NULL.  Change the callers accordingly.

Compile-tested only.

Reported-by: 郭永刚 <guoyonggang@360.cn>
References: http://article.gmane.org/gmane.comp.security.oss.general/17908
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/slip/slip.c

@@ -164,7 +164,7 @@ static int sl_alloc_bufs(struct slip *sl, int mtu)
 	if (cbuff == NULL)
 		goto err_exit;
 	slcomp = slhc_init(16, 16);
-	if (slcomp == NULL)
+	if (IS_ERR(slcomp))
 		goto err_exit;
 #endif
 	spin_lock_bh(&sl->lock);