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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 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) Signed-off-by: Alessio Balsini Cc: rostedt@goodmis.org Cc: mingo@kernel.org Cc: luca.abeni@santannapisa.it Cc: bristot@redhat.com Cc: vpillai@digitalocean.com Cc: kernel-team@android.com Cc: juri.lelli@redhat.com Cc: dietmar.eggemann@arm.com Cc: dvyukov@google.com Cc: tglx@linutronix.de --- 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