net: dsa: felix: fix taprio guard band overflow at 10Mbps with jumbo frames

The DEV_MAC_MAXLEN_CFG register contains a 16-bit value - up to 65535.
Plus 2 * VLAN_HLEN (4), that is up to 65543.

The picos_per_byte variable is the largest when "speed" is lowest -
SPEED_10 = 10. In that case it is (1000000L * 8) / 10 = 800000.

Their product - 52434400000 - exceeds 32 bits, which is a problem,
because apparently, a multiplication between two 32-bit factors is
evaluated as 32-bit before being assigned to a 64-bit variable.
In fact it's a problem for any MTU value larger than 5368.

Cast one of the factors of the multiplication to u64 to force the
multiplication to take place on 64 bits.

Issue found by Coverity.

Fixes: 55a515b1f5a9 ("net: dsa: felix: drop oversized frames with tc-taprio instead of hanging the port")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230613170907.2413559-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/drivers/net/dsa/ocelot/felix_vsc9959.c b/drivers/net/dsa/ocelot/felix_vsc9959.c
index cfb3faeaa5bfa9ee282fe28e3084bf8dbc5d1e5c..d172a3e9736c426516e687e6f99ae1302c2c73d6 100644 (file)
--- a/drivers/net/dsa/ocelot/felix_vsc9959.c
+++ b/drivers/net/dsa/ocelot/felix_vsc9959.c
@@ -1263,7 +1263,7 @@ static void vsc9959_tas_guard_bands_update(struct ocelot *ocelot, int port)
        /* Consider the standard Ethernet overhead of 8 octets preamble+SFD,
         * 4 octets FCS, 12 octets IFG.
         */
-       needed_bit_time_ps = (maxlen + 24) * picos_per_byte;
+       needed_bit_time_ps = (u64)(maxlen + 24) * picos_per_byte;
 
        dev_dbg(ocelot->dev,
                "port %d: max frame size %d needs %llu ps at speed %d\n",