[PATCH v2] panic: prevent panic_timeout * 1000 from overflow

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

From: Changming Liu <charley.ashbringer@gmail.com>

Since panic_timeout is an 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 moving 1000 to the left, also in case i/1000
might never be greater than panic_timeout, change i to
long long so that it strictly has more bits.

Signed-off-by: Changming Liu <charley.ashbringer@gmail.com>
---
Changes in v2:
- change the loop in panic, instead of change the sysctl

 kernel/panic.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/panic.c b/kernel/panic.c
index e2157ca387c8..941848aac0ee 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -178,7 +178,8 @@ void panic(const char *fmt, ...)
 {
 	static char buf[1024];
 	va_list args;
-	long i, i_next = 0, len;
+	long long i;
+	long i_next = 0, len;
 	int state = 0;
 	int old_cpu, this_cpu;
 	bool _crash_kexec_post_notifiers = crash_kexec_post_notifiers;
@@ -315,7 +316,7 @@ void panic(const char *fmt, ...)
 		 */
 		pr_emerg("Rebooting in %d seconds..\n", panic_timeout);
 
-		for (i = 0; i < panic_timeout * 1000; i += PANIC_TIMER_STEP) {
+		for (i = 0; i / 1000 < panic_timeout; i += PANIC_TIMER_STEP) {
 			touch_nmi_watchdog();
 			if (i >= i_next) {
 				i += panic_blink(state ^= 1);
-- 
2.17.1