Right!
Verified that sysctl_sched_dl_period_max and sysctl_sched_dl_period_min values
are now always consistent.
I spent some time in trying to figure out if not having any mutex in
__checkparam_dl() is safe. There can surely happen that "max < min", e.g.:
| | periods
User1 | User2 | checkparam_dl() | sysctl_sched_dl_*
----------|--------------|------------------|-------------------
| | | [x, x]
p_min = 5 | | |
| | | [5, x]
p_max = 5 | | |
| | | [5, 5]
| setattr(p=8) | |
| | p = 8 |
| | [x, 5] |
p_max = 9 | | |
| | | [5, 9]
p_min = 6 | | |
| | | [6, 9]
| | [6, 5] |
----------|--------------|------------------|-------------------
Sharing my thoughts, a "BUG_ON(max < min)" in __checkparam_dl() is then a
guaranteed source of explosions, but the good news is that "if (period < min ||
period > max" in __checkparam_dl() surely fails if "max < min". Also the fact
that, when we are writing the new sysctl_sched_dl_* values, only one is
actually changed at a time, that surely helps to preserve the consistency.
But is that enough?
Well, I couldn't find any counterexample to make __checkparam_dl() pass with
wrong parameters. And the tests I made are happy.
On Thu, Aug 22, 2019 at 06:51:25PM +0200, Peter Zijlstra wrote:
> include/linux/sched/sysctl.h | 7 +++++
> kernel/sched/deadline.c | 58
> +++++++++++++++++++++++++++++++++++++++++--
> kernel/sysctl.c | 14 ++++++++++
> 3 files changed, 77 insertions(+), 2 deletions(-)
>
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> +int sched_dl_period_handler(struct ctl_table *table, int write,
> + void __user *buffer, size_t *lenp,
> + loff_t *ppos)
> +{
> + if (min > 1ULL << DL_SCALE && max > min) {
s/>/>=/
> + WRITE_ONCE(sysctl_sched_dl_period_max, new_max);
> + WRITE_ONCE(sysctl_sched_dl_period_min, new_min);
Besides the inline comment above, this is my ack to your patch.
Otherwise, here follows a slightly more convoluted version of your patch with a
couple of changes to sched_dl_period_handler(), summarized as:
- handle new_table only if writing;
- directly compare the us min and max (saves one multiplication);
- atomic-writes only the sysctl_sched_dl_period_XXX which changed.
M2c.
Thanks!
-Alessio
---
>From cb4481233b57e42ff9dd315811f7919168a28162 Mon Sep 17 00:00:00 2001
From: Peter Zijlstra <[email protected]>
Date: Thu, 22 Aug 2019 18:51:25 +0200
Subject: [PATCH] sched/deadline: Impose global limits on
sched_attr::sched_period
There are two DoS scenarios with SCHED_DEADLINE related to
sched_attr::sched_period:
- since access-control only looks at utilization and density, a very
large period can allow a very large runtime, which in turn can
incur a very large latency to lower priority tasks.
- for very short periods we can end up spending more time programming
the hardware timer than actually running the task.
Mitigate these by imposing limits on the period.
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Signed-off-by: Alessio Balsini <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
---
include/linux/sched/sysctl.h | 7 ++++
kernel/sched/deadline.c | 68 ++++++++++++++++++++++++++++++++++--
kernel/sysctl.c | 14 ++++++++
3 files changed, 87 insertions(+), 2 deletions(-)
diff --git a/include/linux/sched/sysctl.h b/include/linux/sched/sysctl.h
index d4f6215ee03f7..7c8ef07e52133 100644
--- a/include/linux/sched/sysctl.h
+++ b/include/linux/sched/sysctl.h
@@ -56,6 +56,13 @@ int sched_proc_update_handler(struct ctl_table *table, int
write,
extern unsigned int sysctl_sched_rt_period;
extern int sysctl_sched_rt_runtime;
+extern unsigned int sysctl_sched_dl_period_max;
+extern unsigned int sysctl_sched_dl_period_min;
+
+extern int sched_dl_period_handler(struct ctl_table *table, int write,
+ void __user *buffer, size_t *lenp,
+ loff_t *ppos);
+
#ifdef CONFIG_UCLAMP_TASK
extern unsigned int sysctl_sched_uclamp_util_min;
extern unsigned int sysctl_sched_uclamp_util_max;
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 0b9cbfb2b1d4f..c4a6107e055c7 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -2640,6 +2640,59 @@ void __getparam_dl(struct task_struct *p, struct
sched_attr *attr)
attr->sched_flags = dl_se->flags;
}
+/*
+ * Default limits for DL period: on the top end we guard against small util
+ * tasks still getting ridiculous long effective runtimes, on the bottom end we
+ * guard against timer DoS.
+ */
+unsigned int sysctl_sched_dl_period_max = 1 << 22; /* ~4.2 seconds */
+unsigned int sysctl_sched_dl_period_min = 100; /* 100 us */
+
+int sched_dl_period_handler(struct ctl_table *table, int write,
+ void __user *buffer, size_t *lenp,
+ loff_t *ppos)
+{
+ static DEFINE_MUTEX(mutex);
+ int ret;
+
+ mutex_lock(&mutex);
+ if (write) {
+ /*
+ * Use a temporary data structure to store the value read from
+ * userspace. The sysctl_sched_dl_period_{max,min} value will
+ * be updated only if the data is consistent.
+ */
+ struct ctl_table new_table = *table;
+ unsigned int max, min;
+
+ if (new_table.data == &sysctl_sched_dl_period_max) {
+ new_table.data = &max;
+ min = sysctl_sched_dl_period_min;
+ } else {
+ new_table.data = &min;
+ max = sysctl_sched_dl_period_max;
+ }
+
+ ret = proc_douintvec(&new_table, write, buffer, lenp, ppos);
+ if (!ret) {
+ if (min > max ||
+ (u64)min * NSEC_PER_USEC < 1ULL << DL_SCALE) {
+ ret = -EINVAL;
+ } else {
+ unsigned int *src = new_table.data;
+ unsigned int *dst = table->data;
+
+ WRITE_ONCE(*dst, *src);
+ }
+ }
+ } else {
+ ret = proc_douintvec(table, write, buffer, lenp, ppos);
+ }
+ mutex_unlock(&mutex);
+
+ return ret;
+}
+
/*
* This function validates the new parameters of a -deadline task.
* We ask for the deadline not being zero, and greater or equal
@@ -2652,6 +2705,8 @@ void __getparam_dl(struct task_struct *p, struct
sched_attr *attr)
*/
bool __checkparam_dl(const struct sched_attr *attr)
{
+ u64 period, max, min;
+
/* special dl tasks don't actually use any parameter */
if (attr->sched_flags & SCHED_FLAG_SUGOV)
return true;
@@ -2675,12 +2730,21 @@ bool __checkparam_dl(const struct sched_attr *attr)
attr->sched_period & (1ULL << 63))
return false;
+ period = attr->sched_period;
+ if (!period)
+ period = attr->sched_deadline;
+
/* runtime <= deadline <= period (if period != 0) */
- if ((attr->sched_period != 0 &&
- attr->sched_period < attr->sched_deadline) ||
+ if (period < attr->sched_deadline ||
attr->sched_deadline < attr->sched_runtime)
return false;
+ max = (u64)READ_ONCE(sysctl_sched_dl_period_max) * NSEC_PER_USEC;
+ min = (u64)READ_ONCE(sysctl_sched_dl_period_min) * NSEC_PER_USEC;
+
+ if (period < min || period > max)
+ return false;
+
return true;
}
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 078950d9605ba..0d07e4707e9d2 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -442,6 +442,20 @@ static struct ctl_table kern_table[] = {
.mode = 0644,
.proc_handler = sched_rt_handler,
},
+ {
+ .procname = "sched_deadline_period_max_us",
+ .data = &sysctl_sched_dl_period_max,
+ .maxlen = sizeof(unsigned int),
+ .mode = 0644,
+ .proc_handler = sched_dl_period_handler,
+ },
+ {
+ .procname = "sched_deadline_period_min_us",
+ .data = &sysctl_sched_dl_period_min,
+ .maxlen = sizeof(unsigned int),
+ .mode = 0644,
+ .proc_handler = sched_dl_period_handler,
+ },
{
.procname = "sched_rr_timeslice_ms",
.data = &sysctl_sched_rr_timeslice,
--
2.23.0.187.g17f5b7556c-goog
