[PATCH v9 5/7] sched: Handle set_cpus_allowed_ptr() & sched_setaffinity() race

From: Waiman Long
Date: Fri Sep 16 2022 - 14:33:06 EST


Racing is possible between set_cpus_allowed_ptr() and sched_setaffinity()
or between multiple sched_setaffinity() calls from different
CPUs. To resolve these race conditions, we need to update both
user_cpus_ptr and cpus_mask in a single lock critical section instead
of separated ones. This requires moving the user_cpus_ptr update
to set_cpus_allowed_common() by putting the user_mask into the
affinity_context structure.

This patch also changes the handling of the race between the
sched_setaffinity() call and the changing of cpumask of the current
cpuset. In case the new mask conflicts with newly updated cpuset,
the cpus_mask will be reset to the cpuset cpumask and an error value
of -EINVAL will be returned. If a previous user_cpus_ptr value exists,
it will be swapped back in and the new_mask will be further restricted
to what is allowed in the cpumask pointed to by the old user_cpus_ptr.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
kernel/sched/core.c | 44 +++++++++++++++++++++++++++-----------------
kernel/sched/sched.h | 1 +
2 files changed, 28 insertions(+), 17 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b662d8ddc169..c748e56ba254 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2546,6 +2546,12 @@ void set_cpus_allowed_common(struct task_struct *p, struct affinity_context *ctx

cpumask_copy(&p->cpus_mask, ctx->new_mask);
p->nr_cpus_allowed = cpumask_weight(ctx->new_mask);
+
+ /*
+ * Swap in a new user_cpus_ptr if SCA_USER flag set
+ */
+ if (ctx->flags & SCA_USER)
+ swap(p->user_cpus_ptr, ctx->user_mask);
}

static void
@@ -8104,7 +8110,7 @@ __sched_setaffinity(struct task_struct *p, struct affinity_context *ctx)
retval = dl_task_check_affinity(p, new_mask);
if (retval)
goto out_free_new_mask;
-again:
+
retval = __set_cpus_allowed_ptr(p, ctx);
if (retval)
goto out_free_new_mask;
@@ -8116,7 +8122,24 @@ __sched_setaffinity(struct task_struct *p, struct affinity_context *ctx)
* Just reset the cpumask to the cpuset's cpus_allowed.
*/
cpumask_copy(new_mask, cpus_allowed);
- goto again;
+
+ /*
+ * If SCA_USER is set, a 2nd call to __set_cpus_allowed_ptr()
+ * will restore the previous user_cpus_ptr value.
+ *
+ * In the unlikely event a previous user_cpus_ptr exists,
+ * we need to further restrict the mask to what is allowed
+ * by that old user_cpus_ptr.
+ */
+ if (unlikely((ctx->flags & SCA_USER) && ctx->user_mask)) {
+ bool empty = !cpumask_and(new_mask, new_mask,
+ ctx->user_mask);
+
+ if (WARN_ON_ONCE(empty))
+ cpumask_copy(new_mask, cpus_allowed);
+ }
+ __set_cpus_allowed_ptr(p, ctx);
+ retval = -EINVAL;
}

out_free_new_mask:
@@ -8172,25 +8195,12 @@ long sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
cpumask_copy(user_mask, in_mask);
ac = (struct affinity_context){
.new_mask = in_mask,
+ .user_mask = user_mask,
.flags = SCA_USER,
};

retval = __sched_setaffinity(p, &ac);
-
- /*
- * Save in_mask into user_cpus_ptr after a successful
- * __sched_setaffinity() call. pi_lock is used to synchronize
- * changes to user_cpus_ptr.
- */
- if (!retval) {
- unsigned long flags;
-
- /* Use pi_lock to synchronize changes to user_cpus_ptr */
- raw_spin_lock_irqsave(&p->pi_lock, flags);
- swap(p->user_cpus_ptr, user_mask);
- raw_spin_unlock_irqrestore(&p->pi_lock, flags);
- }
- kfree(user_mask);
+ kfree(ac.user_mask);

out_put_task:
put_task_struct(p);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 1927c02f68fa..110e13b7d78b 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2159,6 +2159,7 @@ extern const u32 sched_prio_to_wmult[40];

struct affinity_context {
const struct cpumask *new_mask;
+ struct cpumask *user_mask;
unsigned int flags;
};

--
2.31.1