can: can_dropped_invalid_skb(): ensure an initialized headroom in outgoing CAN sk_buffs

KMSAN sysbot detected a read access to an untinitialized value in the
headroom of an outgoing CAN related sk_buff. When using CAN sockets this
area is filled appropriately - but when using a packet socket this
initialization is missing.

The problematic read access occurs in the CAN receive path which can
only be triggered when the sk_buff is sent through a (virtual) CAN
interface. So we check in the sending path whether we need to perform
the missing initializations.

Fixes: d3b58c47d330d ("can: replace timestamp as unique skb attribute")
Reported-by: syzbot+b02ff0707a97e4e79ebb@syzkaller.appspotmail.com
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Tested-by: Oliver Hartkopp <socketcan@hartkopp.net>
Cc: linux-stable <stable@vger.kernel.org> # >= v4.1
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

diff --git a/include/linux/can/dev.h b/include/linux/can/dev.h
index 9b3c720..5e3d455 100644 (file)
--- a/include/linux/can/dev.h
+++ b/include/linux/can/dev.h
@@ -18,6 +18,7 @@
 #include <linux/can/error.h>
 #include <linux/can/led.h>
 #include <linux/can/netlink.h>
+#include <linux/can/skb.h>
 #include <linux/netdevice.h>
 
 /*
@@ -91,6 +92,36 @@ struct can_priv {
 #define get_can_dlc(i)         (min_t(__u8, (i), CAN_MAX_DLC))
 #define get_canfd_dlc(i)       (min_t(__u8, (i), CANFD_MAX_DLC))
 
+/* Check for outgoing skbs that have not been created by the CAN subsystem */
+static inline bool can_skb_headroom_valid(struct net_device *dev,
+                                         struct sk_buff *skb)
+{
+       /* af_packet creates a headroom of HH_DATA_MOD bytes which is fine */
+       if (WARN_ON_ONCE(skb_headroom(skb) < sizeof(struct can_skb_priv)))
+               return false;
+
+       /* af_packet does not apply CAN skb specific settings */
+       if (skb->ip_summed == CHECKSUM_NONE) {
+               /* init headroom */
+               can_skb_prv(skb)->ifindex = dev->ifindex;
+               can_skb_prv(skb)->skbcnt = 0;
+
+               skb->ip_summed = CHECKSUM_UNNECESSARY;
+
+               /* preform proper loopback on capable devices */
+               if (dev->flags & IFF_ECHO)
+                       skb->pkt_type = PACKET_LOOPBACK;
+               else
+                       skb->pkt_type = PACKET_HOST;
+
+               skb_reset_mac_header(skb);
+               skb_reset_network_header(skb);
+               skb_reset_transport_header(skb);
+       }
+
+       return true;
+}
+
 /* Drop a given socketbuffer if it does not contain a valid CAN frame. */
 static inline bool can_dropped_invalid_skb(struct net_device *dev,
                                          struct sk_buff *skb)
@@ -108,6 +139,9 @@ static inline bool can_dropped_invalid_skb(struct net_device *dev,
        } else
                goto inval_skb;
 
+       if (!can_skb_headroom_valid(dev, skb))
+               goto inval_skb;
+
        return false;
 
 inval_skb: