Syzbot reported a WARNING in taprio_get_start_time().

When link speed is 470589 or greater, q->picos_per_byte becomes too
small, causing length_to_duration(q, ETH_ZLEN) to return zero.

This zero value leads to validation failures in fill_sched_entry() and
parse_taprio_schedule(), allowing arbitrary values to be assigned to
entry->interval and cycle_time. As a result, sched->cycle can become zero.

Since SPEED_800000 is the largest defined speed in
include/uapi/linux/ethtool.h, this issue can occur in realistic scenarios.

To ensure length_to_duration() returns a non-zero value for minimum-sized
Ethernet frames (ETH_ZLEN = 60), picos_per_byte must be at least 17
(60 * 17 > PSEC_PER_NSEC which is 1000).

This patch enforces a minimum value of 17 for picos_per_byte when the
calculated value would be lower.

Fixes: 68ce6688a5ba ("net: sched: taprio: Fix potential integer overflow in taprio_set_picos_per_byte")
Reported-by: syzbot+398e1ee4ca2cac05fddb@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=398e1ee4ca2cac05fddb
Signed-off-by: Takamitsu Iwai <redacted>
---
 net/sched/sch_taprio.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
index 2b14c81a87e5..bcfdb9446657 100644
--- a/net/sched/sch_taprio.c
+++ b/net/sched/sch_taprio.c

@@ -43,6 +43,12 @@ static struct static_key_false taprio_have_working_mqprio;
 #define TAPRIO_SUPPORTED_FLAGS \
 	(TCA_TAPRIO_ATTR_FLAG_TXTIME_ASSIST | TCA_TAPRIO_ATTR_FLAG_FULL_OFFLOAD)
 #define TAPRIO_FLAGS_INVALID U32_MAX
+/* Minimum value for picos_per_byte to ensure non-zero duration
+ * for minimum-sized Ethernet frames (ETH_ZLEN = 60).
+ * 60 * 17 > PSEC_PER_NSEC (1000)
+ */
+#define TAPRIO_PICOS_PER_BYTE_MIN 17
+
 
 struct sched_entry {
 	/* Durations between this GCL entry and the GCL entry where the

@@ -1299,7 +1305,7 @@ static void taprio_set_picos_per_byte(struct net_device *dev,
 		speed = ecmd.base.speed;
 
 skip:
-	picos_per_byte = (USEC_PER_SEC * 8) / speed;
+	picos_per_byte = max((USEC_PER_SEC * 8) / speed, TAPRIO_PICOS_PER_BYTE_MIN);
 
 	atomic64_set(&q->picos_per_byte, picos_per_byte);
 	netdev_dbg(dev, "taprio: set %s's picos_per_byte to: %lld, linkspeed: %d\n",

-- 
2.39.5 (Apple Git-154)