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net/sched: taprio: enforce minimum value for picos_per_byte

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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>
Link: https://patch.msgid.link/20250728173149.45585-1-takamitz@amazon.co.jp
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Takamitsu Iwai 2025-07-29 02:31:49 +09:00 committed by Jakub Kicinski
parent d46e51f1c7
commit ae8508b25d
1 changed files with 18 additions and 3 deletions
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21
net/sched/sch_taprio.c
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@ -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)