UBUNTU: SAUCE: Fix posix clock speculation mitigation backport

BugLink: https://launchpad.net/bugs/1847189

The Ubuntu Xenial backport of upstream commit 19b558db
("posix-timers: Protect posix clock array access against speculation")
incorrectly dropped the NULL check on the .clock_getres function
pointer. Readd the NULL check while still protecting against
side-channel speculation attacks when indexing into the posix_clocks
array to perform that NULL check.

The NULL check protects against a denial of service (system crash) or
possible arbitrary code execution that can be triggered by
clock_gettime(10, 0), as pointed out by Vitaly Nikolenko.

Fixes: eb4a3a43 ("posix-timers: Protect posix clock array access against speculation")
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Andrea Righi <andrea.righi@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

kernel/time/posix-timers.c

@@ -606,7 +606,11 @@ static struct k_clock *clockid_to_kclock(const clockid_t id)
 	if (id >= MAX_CLOCKS)
 		return NULL;
 
-	return &posix_clocks[array_index_nospec(idx, MAX_CLOCKS)];
+	idx = array_index_nospec(idx, MAX_CLOCKS);
+	if (!posix_clocks[idx].clock_getres)
+		return NULL;
+
+	return &posix_clocks[idx];
 }
 
 static int common_timer_create(struct k_itimer *new_timer)