sched/core: 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=7281c8dec8a87685cb54d503d8cceef5a0fc2fdd
Commit:     7281c8dec8a87685cb54d503d8cceef5a0fc2fdd
Parent:     b5bf9a90bbebffba888c9144c5a8a10317b04064
Author:     Peter Zijlstra <peterz at infradead.org>
AuthorDate: Fri Apr 20 14:29:51 2018 +0200
Committer:  Ingo Molnar <mingo at kernel.org>
CommitDate: Sat May 5 08:32:36 2018 +0200

    sched/core: Fix possible Spectre-v1 indexing for sched_prio_to_weight[]
    
    > kernel/sched/core.c:6921 cpu_weight_nice_write_s64() warn: potential spectre issue 'sched_prio_to_weight'
    
    Userspace controls @nice, so sanitize the value before using it to
    index an array.
    
    Reported-by: Dan Carpenter <dan.carpenter at oracle.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz at infradead.org>
    Cc: <stable at kernel.org>
    Cc: Linus Torvalds <torvalds at linux-foundation.org>
    Cc: Mike Galbraith <efault at gmx.de>
    Cc: Peter Zijlstra <peterz at infradead.org>
    Cc: Thomas Gleixner <tglx at linutronix.de>
    Cc: linux-kernel at vger.kernel.org
    Signed-off-by: Ingo Molnar <mingo at kernel.org>
---
 kernel/sched/core.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ffde9eebc846..092f7c4de903 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8,6 +8,7 @@
 #include "sched.h"
 
 #include <linux/kthread.h>
+#include <linux/nospec.h>
 
 #include <asm/switch_to.h>
 #include <asm/tlb.h>
@@ -6923,11 +6924,15 @@ static int cpu_weight_nice_write_s64(struct cgroup_subsys_state *css,
 				     struct cftype *cft, s64 nice)
 {
 	unsigned long weight;
+	int idx;
 
 	if (nice < MIN_NICE || nice > MAX_NICE)
 		return -ERANGE;
 
-	weight = sched_prio_to_weight[NICE_TO_PRIO(nice) - MAX_RT_PRIO];
+	idx = NICE_TO_PRIO(nice) - MAX_RT_PRIO;
+	idx = array_index_nospec(idx, 40);
+	weight = sched_prio_to_weight[idx];
+
 	return sched_group_set_shares(css_tg(css), scale_load(weight));
 }
 #endif