[PATCH 3.16 261/366] sched/core: Fix possible Spectre-v1 indexing for sched_prio_to_weight[]

[Ben Hutchings]

3.16.60-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

commit 7281c8dec8a87685cb54d503d8cceef5a0fc2fdd upstream.

> 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@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Mike Galbraith <efault@xxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
[bwh: Backported to 3.16: Vulnerable array lookup is in set_load_weight()]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -74,6 +74,7 @@
 #include <linux/binfmts.h>
 #include <linux/context_tracking.h>
 #include <linux/compiler.h>
+#include <linux/nospec.h>
 
 #include <asm/switch_to.h>
 #include <asm/tlb.h>
@@ -820,6 +821,8 @@ static void set_load_weight(struct task_
 		return;
 	}
 
+	prio = array_index_nospec(prio, 40);
+
 	load->weight = scale_load(prio_to_weight[prio]);
 	load->inv_weight = prio_to_wmult[prio];
 }