[PATCH v3 net] net/sched: taprio: enforce minimum value for picos_per_byte
From: Takamitsu Iwai <hidden>
Date: 2025-07-28 17:32:14
Subsystem:
cbs/etf/taprio qdiscs, networking [general], tc subsystem, the rest · Maintainers:
Vinicius Costa Gomes, "David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Jamal Hadi Salim, Jiri Pirko, Linus Torvalds
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 <redacted>
---
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/ (local)
- 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/ (local)
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)