[PATCH v3 net] net/sched: taprio: enforce minimum value for picos_per_byte

[Takamitsu Iwai <takamitz@amazon.co.jp>]

Syzbot reported a WARNING in taprio_get_start_time().

When link speed is 470,589 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, and adds a warning message to inform
users that scheduling accuracy may be affected at very high link speeds.

Fixes: fb66df20a720 ("net/sched: taprio: extend minimum interval restriction to entire cycle too")
Reported-by: syzbot+398e1ee4ca2cac05fddb@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=398e1ee4ca2cac05fddb
Signed-off-by: Takamitsu Iwai <takamitz@amazon.co.jp>
---
Changes:
  v3:
    - Remove unnecessary blank line.
    - Add NL_SET_ERR_MSG_FMT_MOD() to show warning directly to the users
      when taprio_set_picos_per_byte() is called from taprio_change().

  v2: https://lore.kernel.org/all/20250726010815.20198-1-takamitz@amazon.co.jp/
    - Add pr_warn() for users to inform link speed is too high for scheduling.
    - Correct fixes tag to indicate appropriate commit.

  v1: https://lore.kernel.org/all/20250724181345.40961-1-takamitz@amazon.co.jp/

 net/sched/sch_taprio.c | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
index e759e43ad27e..39b735386996 100644
--- a/net/sched/sch_taprio.c
+++ b/net/sched/sch_taprio.c
@@ -43,6 +43,11 @@ 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
@@ -1284,7 +1289,8 @@ static void taprio_start_sched(struct Qdisc *sch,
 }
 
 static void taprio_set_picos_per_byte(struct net_device *dev,
-				      struct taprio_sched *q)
+				      struct taprio_sched *q,
+				      struct netlink_ext_ack *extack)
 {
 	struct ethtool_link_ksettings ecmd;
 	int speed = SPEED_10;
@@ -1300,6 +1306,15 @@ static void taprio_set_picos_per_byte(struct net_device *dev,
 
 skip:
 	picos_per_byte = (USEC_PER_SEC * 8) / speed;
+	if (picos_per_byte < TAPRIO_PICOS_PER_BYTE_MIN) {
+		if (!extack)
+			pr_warn("Link speed %d is too high. Schedule may be inaccurate.\n",
+				speed);
+		NL_SET_ERR_MSG_FMT_MOD(extack,
+				       "Link speed %d is too high. Schedule may be inaccurate.",
+				       speed);
+		picos_per_byte = 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",
@@ -1324,7 +1339,7 @@ static int taprio_dev_notifier(struct notifier_block *nb, unsigned long event,
 		if (dev != qdisc_dev(q->root))
 			continue;
 
-		taprio_set_picos_per_byte(dev, q);
+		taprio_set_picos_per_byte(dev, q, NULL);
 
 		stab = rtnl_dereference(q->root->stab);
 
@@ -1844,7 +1859,7 @@ static int taprio_change(struct Qdisc *sch, struct nlattr *opt,
 	q->flags = taprio_flags;
 
 	/* Needed for length_to_duration() during netlink attribute parsing */
-	taprio_set_picos_per_byte(dev, q);
+	taprio_set_picos_per_byte(dev, q, extack);
 
 	err = taprio_parse_mqprio_opt(dev, mqprio, extack, q->flags);
 	if (err < 0)
-- 
2.39.5 (Apple Git-154)