sched/autogroup: Fix possible Spectre-v1 indexing for sched_prio_to_weight[]

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Gitweb:     http://git.infradead.org/?p=mtd-2.6.git;a=commit;h=354d7793070611b4df5a79fbb0f12752d0ed0cc5
Commit:     354d7793070611b4df5a79fbb0f12752d0ed0cc5
Parent:     7281c8dec8a87685cb54d503d8cceef5a0fc2fdd
Author:     Peter Zijlstra <peterz at infradead.org>
AuthorDate: Fri Apr 20 15:03:45 2018 +0200
Committer:  Ingo Molnar <mingo at kernel.org>
CommitDate: Sat May 5 08:34:42 2018 +0200

    sched/autogroup: Fix possible Spectre-v1 indexing for sched_prio_to_weight[]
    
    > kernel/sched/autogroup.c:230 proc_sched_autogroup_set_nice() warn: potential spectre issue 'sched_prio_to_weight'
    
    Userspace controls @nice, sanitize the array index.
    
    Reported-by: Dan Carpenter <dan.carpenter at oracle.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz at infradead.org>
    Cc: Linus Torvalds <torvalds at linux-foundation.org>
    Cc: Peter Zijlstra <peterz at infradead.org>
    Cc: Thomas Gleixner <tglx at linutronix.de>
    Cc: <stable at kernel.org>
    Signed-off-by: Ingo Molnar <mingo at kernel.org>
---
 kernel/sched/autogroup.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/autogroup.c b/kernel/sched/autogroup.c
index 6be6c575b6cd..2d4ff5353ded 100644
--- a/kernel/sched/autogroup.c
+++ b/kernel/sched/autogroup.c
@@ -2,6 +2,7 @@
 /*
  * Auto-group scheduling implementation:
  */
+#include <linux/nospec.h>
 #include "sched.h"
 
 unsigned int __read_mostly sysctl_sched_autogroup_enabled = 1;
@@ -209,7 +210,7 @@ int proc_sched_autogroup_set_nice(struct task_struct *p, int nice)
 	static unsigned long next = INITIAL_JIFFIES;
 	struct autogroup *ag;
 	unsigned long shares;
-	int err;
+	int err, idx;
 
 	if (nice < MIN_NICE || nice > MAX_NICE)
 		return -EINVAL;
@@ -227,7 +228,9 @@ int proc_sched_autogroup_set_nice(struct task_struct *p, int nice)
 
 	next = HZ / 10 + jiffies;
 	ag = autogroup_task_get(p);
-	shares = scale_load(sched_prio_to_weight[nice + 20]);
+
+	idx = array_index_nospec(nice + 20, 40);
+	shares = scale_load(sched_prio_to_weight[idx]);
 
 	down_write(&ag->lock);
 	err = sched_group_set_shares(ag->tg, shares);