[PATCH] sched: Fix __sched_setscheduler() nice test

[Peter Zijlstra]

Subject: sched: Fix __sched_setscheduler() nice test

With the introduction of sched_attr::sched_nice we need to check if
we've got permission to actually change the nice value.

Daniel found that can_nice() would always fail; and upon inspection it
turns out that can_nice() only tests to see if we can lower the nice
value, but it doesn't validate if we're lowering or not.

Therefore amend the test to only call can_nice() when we lower the nice
value.

Cc: raistlin@xxxxxxxx
Cc: juri.lelli@xxxxxxxxx
Fixes: d50dde5a10f3 ("sched: Add new scheduler syscalls to support an extended scheduling parameters ABI")
Reported-and-Tested-by: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx> 
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
---
 kernel/sched/core.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 93a2836b6220..36c951b7eef8 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3296,7 +3296,8 @@ static int __sched_setscheduler(struct task_struct *p,
 	 */
 	if (user && !capable(CAP_SYS_NICE)) {
 		if (fair_policy(policy)) {
-			if (!can_nice(p, attr->sched_nice))
+			if (attr->sched_nice < TASK_NICE(p) &&
+			    !can_nice(p, attr->sched_nice))
 				return -EPERM;
 		}
 
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