[OpenWrt-Devel] [PATCH] [netifd] vlan: Array out of bounds in snprintf for vlans

cshored at thecshore.com cshored at thecshore.com
Tue Jan 30 13:16:49 EST 2018

From: "Daniel F. Dickinson" <cshored at thecshore.com>

Detected during a side project.  Not a brilliant fix, but it
gets the job done for now.  *very* lightly tested, more
for your information than anything else.

Array out-of-bounds condition can occur because vlan
device name is constructed from device name (size IFNAMSIZ)
plus the ASCII decimal representation of the vlan id plus
a dot, but the target can only be IFNAMSIZ.  We fix this
by using fields widths (and make sure we don't truncate
more of the orogin device name than we must).

Signed-off-by: Daniel F. Dickinson <cshored at thecshore.com>
 vlan.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/vlan.c b/vlan.c
index 067f624..44852f4 100644
--- a/vlan.c
+++ b/vlan.c
@@ -63,10 +63,17 @@ static int vlan_set_device_state(struct device *dev, bool up)
 static void vlan_dev_set_name(struct vlan_device *vldev, struct device *dev)
-	char name[IFNAMSIZ];
+	char name[IFNAMSIZ + 1];
+	char devnum[5];
+	int i, j = 0;
 	vldev->dev.hidden = dev->hidden;
-	snprintf(name, IFNAMSIZ, "%s.%d", dev->ifname, vldev->id);
+	snprintf(devnum, 4, "%d", vldev->id);
+	i = strnlen(devnum, 4);
+	j = IFNAMSIZ - i;
+	strncpy(name, dev->ifname, j);
+	strncat(name, ".", 1);
+	strncat(name, devnum, i);
 	device_set_ifname(&vldev->dev, name);
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