[PATCH v3] panic: prevent panic_timeout * 1000 from overflow

[Changming Liu <charley.ashbringer@gmail.com>]

Since panic_timeout is an s32 integer passed in through sysctl, the
loop boundary panic_timeout * 1000 could overflow and result in
a zero-delay panic when panic_timeout is greater than INT_MAX/1000.

Fix this by elevating the precision of the loop boundary via 
assigning the result to a u64 integer, also in case the loop 
counter i might never be greater than u64 timeout = panic_timeout*1000,
elevate its precision to u64(timer) as well. The same applies to
timer_next replacing i_next which is initialized to 0.

Signed-off-by: Changming Liu <charley.ashbringer@gmail.com>
---
Changes in v3:
- change the loop in panic, instead of change the sysctl
- avoid using 64-bit division, doing 64-bit mult instead

 kernel/panic.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/kernel/panic.c b/kernel/panic.c
index e2157ca..ef6cd57 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -313,13 +313,16 @@ void panic(const char *fmt, ...)
 		 * Delay timeout seconds before rebooting the machine.
 		 * We can't use the "normal" timers since we just panicked.
 		 */
+		u64 timeout = panic_timeout * 1000ULL;	/* avoid overflow */
+		u64 timer, timer_next = 0;
+
 		pr_emerg("Rebooting in %d seconds..\n", panic_timeout);
 
-		for (i = 0; i < panic_timeout * 1000; i += PANIC_TIMER_STEP) {
+		for (timer = 0; timer < timeout; timer += PANIC_TIMER_STEP) {
 			touch_nmi_watchdog();
-			if (i >= i_next) {
-				i += panic_blink(state ^= 1);
-				i_next = i + 3600 / PANIC_BLINK_SPD;
+			if (timer >= timer_next) {
+				timer += panic_blink(state ^= 1);
+				timer_next = timer + 3600 / PANIC_BLINK_SPD;
 			}
 			mdelay(PANIC_TIMER_STEP);
 		}
-- 
2.7.4