posix-timers: Protect posix clock array access against speculation

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@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: stable@vger.kernel.org
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Link: https://lkml.kernel.org/r/alpine.DEB.2.21.1802151718320.1296@nanos.tec.linutronix.de

CVE-2017-5753

(backported from commit 19b558db)
[juergh: Context adjustments.]
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

kernel/time/posix-timers.c

@@ -48,6 +48,7 @@
 #include <linux/workqueue.h>
 #include <linux/export.h>
 #include <linux/hashtable.h>
+#include <linux/nospec.h>
 
 #include "timekeeping.h"
 
@@ -595,13 +596,17 @@ static void release_posix_timer(struct k_itimer *tmr, int it_id_set)
 
 static struct k_clock *clockid_to_kclock(const clockid_t id)
 {
-	if (id < 0)
+	clockid_t idx = id;
+
+	if (id < 0) {
 		return (id & CLOCKFD_MASK) == CLOCKFD ?
 			&clock_posix_dynamic : &clock_posix_cpu;
+	}
 
-	if (id >= MAX_CLOCKS || !posix_clocks[id].clock_getres)
+	if (id >= MAX_CLOCKS)
 		return NULL;
-	return &posix_clocks[id];
+
+	return &posix_clocks[array_index_nospec(idx, MAX_CLOCKS)];
 }
 
 static int common_timer_create(struct k_itimer *new_timer)