[PATCH 3.16 389/410] posix-timers: Protect posix clock array access against speculation

[Ben Hutchings]

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

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

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

commit 19b558db12f9f4e45a22012bae7b4783e62224da upstream.

The clockid argument of clockid_to_kclock() comes straight from user space
via various syscalls and is used as index into the posix_clocks array.

Protect it against spectre v1 array out of bounds speculation. Remove the
redundant check for !posix_clock[id] as this is another source for
speculation and does not provide any advantage over the return
posix_clock[id] path which returns NULL in that case anyway.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: Rasmus Villemoes <rasmus.villemoes@xxxxxxxxx>
Cc: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: David Woodhouse <dwmw@xxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/alpine.DEB.2.21.1802151718320.1296@xxxxxxxxxxxxxxxxxxxxxxx
[bwh: Backported to 3.16:
 - Move the test of the clock_getres field below the lookup using
   array_index_nospec()
 - Adjust filename, context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/kernel/posix-timers.c
+++ b/kernel/posix-timers.c
@@ -48,6 +48,7 @@
 #include <linux/workqueue.h>
 #include <linux/export.h>
 #include <linux/hashtable.h>
+#include <linux/nospec.h>
 
 /*
  * Management arrays for POSIX timers. Timers are now kept in static hash table
@@ -578,13 +579,21 @@ static void release_posix_timer(struct k
 
 static struct k_clock *clockid_to_kclock(const clockid_t id)
 {
-	if (id < 0)
+	clockid_t idx = id;
+	struct k_clock *kc;
+
+	if (id < 0) {
 		return (id & CLOCKFD_MASK) == CLOCKFD ?
 			&clock_posix_dynamic : &clock_posix_cpu;
+	}
+
+	if (id >= ARRAY_SIZE(posix_clocks))
+		return NULL;
 
-	if (id >= MAX_CLOCKS || !posix_clocks[id].clock_getres)
+	kc = &posix_clocks[array_index_nospec(idx, ARRAY_SIZE(posix_clocks))];
+	if (!kc->clock_getres)
 		return NULL;
-	return &posix_clocks[id];
+	return kc;
 }
 
 static int common_timer_create(struct k_itimer *new_timer)