[PATCH] blk-iocost: avoid out of bounds shift

[Rik van Riel]

UBSAN catches undefined behavior in blk-iocost, where sometimes
iocg->delay is shifted right by a number that is too large,
resulting in undefined behavior on some architectures.

[  186.556576] ------------[ cut here ]------------
UBSAN: shift-out-of-bounds in block/blk-iocost.c:1366:23
shift exponent 64 is too large for 64-bit type 'u64' (aka 'unsigned long long')
CPU: 16 PID: 0 Comm: swapper/16 Tainted: G S          E    N 6.9.0-0_fbk700_debug_rc2_kbuilder_0_gc85af715cac0 #1
Hardware name: Quanta Twin Lakes MP/Twin Lakes Passive MP, BIOS F09_3A23 12/08/2020
Call Trace:
 <IRQ>
 dump_stack_lvl+0x8f/0xe0
 __ubsan_handle_shift_out_of_bounds+0x22c/0x280
 iocg_kick_delay+0x30b/0x310
 ioc_timer_fn+0x2fb/0x1f80
 __run_timer_base+0x1b6/0x250
..

Avoid that undefined behavior by simply taking the
"delay = 0" branch if the shift is too large.

I am not sure what the symptoms of an undefined value
delay will be, but I suspect it could be more than a
little annoying to debug.

Signed-off-by: Rik van Riel <riel@xxxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Josef Bacik <josef@xxxxxxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
---
 block/blk-iocost.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/block/blk-iocost.c b/block/blk-iocost.c
index 9a85bfbbc45a..baa20c85799d 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 +1362,9 @@ 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;
-	if (iocg->delay)
-		delay = iocg->delay >> div64_u64(tdelta, USEC_PER_SEC);
+	shift = div64_u64(tdelta, USEC_PER_SEC);
+	if (iocg->delay && shift < BITS_PER_LONG)
+		delay = iocg->delay >> shift;
 	else
 		delay = 0;
 
-- 
2.42.0