net: ppp: fix creating PPP pass and active filters

Commit 568f194 ("net: ppp: use
sk_unattached_filter api") inadvertently changed the logic when setting
PPP pass and active filters. This applies to both the generic PPP subsystem
implemented by drivers/net/ppp/ppp_generic.c and the ISDN PPP subsystem
implemented by drivers/isdn/i4l/isdn_ppp.c. The original code in ppp_ioctl()
(or isdn_ppp_ioctl(), resp.) handling PPPIOCSPASS and PPPIOCSACTIVE allowed to
remove a pass/active filter previously set by using a filter of length zero.
However, with the new code this is not possible anymore as this case is not
explicitly checked for, which leads to passing NULL as a filter to
sk_unattached_filter_create(). This results in returning EINVAL to the caller.

Additionally, the variables ppp->pass_filter and ppp->active_filter (or
is->pass_filter and is->active_filter, resp.) are not reset to NULL, although
the filters they point to may have been destroyed by
sk_unattached_filter_destroy(), so in this EINVAL case dangling pointers are
left behind (provided the pointers were previously non-NULL).

This patch corrects both problems by checking whether the filter passed is
empty or non-empty, and prevents sk_unattached_filter_create() from being
called in the first case. Moreover, the pointers are always reset to NULL
as soon as sk_unattached_filter_destroy() returns.

Signed-off-by: Christoph Schulz <develop@kristov.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/isdn/i4l/isdn_ppp.c

@@ -638,9 +638,15 @@ isdn_ppp_ioctl(int min, struct file *file, unsigned int cmd, unsigned long arg)
 		fprog.len = len;
 		fprog.filter = code;
 
-		if (is->pass_filter)
+		if (is->pass_filter) {
 			sk_unattached_filter_destroy(is->pass_filter);
-		err = sk_unattached_filter_create(&is->pass_filter, &fprog);
+			is->pass_filter = NULL;
+		}
+		if (fprog.filter != NULL)
+			err = sk_unattached_filter_create(&is->pass_filter,
+							  &fprog);
+		else
+			err = 0;
 		kfree(code);
 
 		return err;
@@ -657,9 +663,15 @@ isdn_ppp_ioctl(int min, struct file *file, unsigned int cmd, unsigned long arg)
 		fprog.len = len;
 		fprog.filter = code;
 
-		if (is->active_filter)
+		if (is->active_filter) {
 			sk_unattached_filter_destroy(is->active_filter);
-		err = sk_unattached_filter_create(&is->active_filter, &fprog);
+			is->active_filter = NULL;
+		}
+		if (fprog.filter != NULL)
+			err = sk_unattached_filter_create(&is->active_filter,
+							  &fprog);
+		else
+			err = 0;
 		kfree(code);
 
 		return err;

drivers/net/ppp/ppp_generic.c

@@ -757,10 +757,15 @@ static long ppp_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 			};
 
 			ppp_lock(ppp);
-			if (ppp->pass_filter)
+			if (ppp->pass_filter) {
 				sk_unattached_filter_destroy(ppp->pass_filter);
-			err = sk_unattached_filter_create(&ppp->pass_filter,
-							  &fprog);
+				ppp->pass_filter = NULL;
+			}
+			if (fprog.filter != NULL)
+				err = sk_unattached_filter_create(&ppp->pass_filter,
+								  &fprog);
+			else
+				err = 0;
 			kfree(code);
 			ppp_unlock(ppp);
 		}
@@ -778,10 +783,15 @@ static long ppp_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 			};
 
 			ppp_lock(ppp);
-			if (ppp->active_filter)
+			if (ppp->active_filter) {
 				sk_unattached_filter_destroy(ppp->active_filter);
-			err = sk_unattached_filter_create(&ppp->active_filter,
-							  &fprog);
+				ppp->active_filter = NULL;
+			}
+			if (fprog.filter != NULL)
+				err = sk_unattached_filter_create(&ppp->active_filter,
+								  &fprog);
+			else
+				err = 0;
 			kfree(code);
 			ppp_unlock(ppp);
 		}