[PATCH] blk-iocost: Fix shift-out-of-bounds in iocg_kick_delay()

[Breno Leitao]

When running iocg_kick_delay(), iocg->delay_at could be way behind "now",
which causes a huge tdelta difference.

The tdelta value is used to shift some bits around, raising the
following UBSAN splat:

	UBSAN: shift-out-of-bounds in block/blk-iocost.c:1366:23

Debugging this, these are some values I got in my machine with 6.9-rc3.

 now = 3626064295
 iocg->delay_at = 3275794093

Fix this by validating that the shift if valid, otherwise bail out,
similarly to commit 2a427b49d029 ("blk-iocost: Fix an UBSAN
shift-out-of-bounds warning")

Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
---
 block/blk-iocost.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/block/blk-iocost.c b/block/blk-iocost.c
index 9a85bfbbc45a..398fe19db4ca 100644
--- a/block/blk-iocost.c
+++ b/block/blk-iocost.c
@@ -1347,7 +1347,7 @@ static bool iocg_kick_delay(struct ioc_gq *iocg, struct ioc_now *now)
 {
 	struct ioc *ioc = iocg->ioc;
 	struct blkcg_gq *blkg = iocg_to_blkg(iocg);
-	u64 tdelta, delay, new_delay;
+	u64 tdelta, delay, new_delay, shift;
 	s64 vover, vover_pct;
 	u32 hwa;
 
@@ -1362,8 +1362,13 @@ static bool iocg_kick_delay(struct ioc_gq *iocg, struct ioc_now *now)
 
 	/* calculate the current delay in effect - 1/2 every second */
 	tdelta = now->now - iocg->delay_at;
+
+	shift = div64_u64(tdelta, USEC_PER_SEC);
+	if (shift >= BITS_PER_LONG)
+		return false;
+
 	if (iocg->delay)
-		delay = iocg->delay >> div64_u64(tdelta, USEC_PER_SEC);
+		delay = iocg->delay >> shift;
 	else
 		delay = 0;
 
-- 
2.43.0