drm/repaper: fix integer overflows in repeat functions

There are conditions, albeit somewhat unlikely, under which right hand
expressions, calculating the end of time period in functions like
repaper_frame_fixed_repeat(), may overflow.

For instance, if 'factor10x' in repaper_get_temperature() is high
enough (170), as is 'epd->stage_time' in repaper_probe(), then the
resulting value of 'end' will not fit in unsigned int expression.

Mitigate this by casting 'epd->factored_stage_time' to wider type before
any multiplication is done.

Found by Linux Verification Center (linuxtesting.org) with static
analysis tool SVACE.

Fixes: 3589211 ("drm/tinydrm: Add RePaper e-ink driver")
Cc: [email protected]
Signed-off-by: Nikita Zhandarovich <[email protected]>
Signed-off-by: Alex Lanzano <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

Author: Nikita Zhandarovich

drivers/gpu/drm/tiny/repaper.c

@@ -456,7 +456,7 @@ static void repaper_frame_fixed_repeat(struct repaper_epd *epd, u8 fixed_value,
 				       enum repaper_stage stage)
 {
 	u64 start = local_clock();
-	u64 end = start + (epd->factored_stage_time * 1000 * 1000);
+	u64 end = start + ((u64)epd->factored_stage_time * 1000 * 1000);
 
 	do {
 		repaper_frame_fixed(epd, fixed_value, stage);
@@ -467,7 +467,7 @@ static void repaper_frame_data_repeat(struct repaper_epd *epd, const u8 *image,
 				      const u8 *mask, enum repaper_stage stage)
 {
 	u64 start = local_clock();
-	u64 end = start + (epd->factored_stage_time * 1000 * 1000);
+	u64 end = start + ((u64)epd->factored_stage_time * 1000 * 1000);
 
 	do {
 		repaper_frame_data(epd, image, mask, stage);