[PATCH] sched/autogroup: Serialize nice rate-limit updates

[Hui Su <sh_def@163.com>]

proc_sched_autogroup_set_nice() uses a function-static timestamp to
rate-limit the expensive sched_group_set_shares() operation.

The timestamp check and update are not serialized. Two concurrent
unprivileged writers can therefore both observe an expired timestamp,
update it, and proceed to sched_group_set_shares(), defeating the
intended 100 ms rate limit.

Serialize the timestamp check and update with a dedicated spinlock.
Evaluate the capability before entering the lock and keep the heavy
autogroup operation outside the critical section.

Fixes: 5091faa449ee ("sched: Add 'autogroup' scheduling feature: automated per session task groups")
Signed-off-by: Hui Su <sh_def@163.com>
---
Testing:
- x86_64 KVM KCSAN forced-overlap reproducer reported concurrent writes
  to the shared timestamp from proc_sched_autogroup_set_nice() on CPUs
  0 and 1.
- x86_64 KVM rate-limit oracle observed two pre-fix writes within 20 ms
  and no post-fix pair within a 100 ms window.
---
 kernel/sched/autogroup.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/autogroup.c b/kernel/sched/autogroup.c
index e380cf9372bb..3469a52dd255 100644
--- a/kernel/sched/autogroup.c
+++ b/kernel/sched/autogroup.c
@@ -230,11 +230,14 @@ __setup("noautogroup", setup_autogroup);
 
 #ifdef CONFIG_PROC_FS
 
+static DEFINE_SPINLOCK(autogroup_nice_lock);
+
 int proc_sched_autogroup_set_nice(struct task_struct *p, int nice)
 {
 	static unsigned long next = INITIAL_JIFFIES;
 	struct autogroup *ag;
 	unsigned long shares;
+	bool admin;
 	int err, idx;
 
 	if (nice < MIN_NICE || nice > MAX_NICE)
@@ -247,11 +250,17 @@ int proc_sched_autogroup_set_nice(struct task_struct *p, int nice)
 	if (nice < 0 && !can_nice(current, nice))
 		return -EPERM;
 
-	/* This is a heavy operation, taking global locks.. */
-	if (!capable(CAP_SYS_ADMIN) && time_before(jiffies, next))
+	admin = capable(CAP_SYS_ADMIN);
+
+	/* Rate-limit this heavy operation, which takes global locks. */
+	spin_lock(&autogroup_nice_lock);
+	if (!admin && time_before(jiffies, next)) {
+		spin_unlock(&autogroup_nice_lock);
 		return -EAGAIN;
+	}
 
 	next = HZ / 10 + jiffies;
+	spin_unlock(&autogroup_nice_lock);
 	ag = autogroup_task_get(p);
 
 	idx = array_index_nospec(nice + 20, 40);

base-commit: 6f8319e3e9a44dd537d17f41565a8453c560a581
-- 
2.55.0