[PATCH v2] HID: pidff: bound the unit exponent rescaling loop

["Shengzhuo Wei" <me@cherr.cc>]

pidff_rescale_time() walks the device-provided unit exponent one step
at a time, allowing extreme values to cause billions of iterations
while ff->mutex is held. In a UHID test with an INT_MIN exponent,
EVIOCSFF took 3.1 to 4.3 seconds to upload a finite-duration effect.

Stop rescaling once scaled_time becomes zero, since further
multiplication or division by 10 cannot change the result. The u32
value becomes zero within 32 multiplications by 10, and the u16
input within 5 divisions. This bounds the loops while preserving
their results for all exponents, including wraparound.

Also warn during initialization if the duration field's unit exponent
is outside the range -8 to 0. This helps identify unusual device
descriptors without logging on every effect upload or changing the
rescaling result.

Fixes: 8713107221a8 ("HID: pidff: Rescale time values to match field units")
Reviewed-by: Tomasz Pakuła <tomasz.pakula.oficjalny@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Shengzhuo Wei <me@cherr.cc>
---
v1: https://lore.kernel.org/r/20261029-b4-pidff-rescale-exponent-v1-1-3ba54accf767@cherr.cc

Changes in v2:
- Following Tomasz's suggestion, warn during initialization when
  the duration field's unit exponent is outside -8..0.
- Add Tomasz's Reviewed-by.
---
 drivers/hid/usbhid/hid-pidff.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/hid/usbhid/hid-pidff.c b/drivers/hid/usbhid/hid-pidff.c
index 22951b7ecd17..4540f49fe6e3 100644
--- a/drivers/hid/usbhid/hid-pidff.c
+++ b/drivers/hid/usbhid/hid-pidff.c
@@ -354,9 +354,9 @@ static u32 pidff_rescale_time(u16 time, struct hid_field *field)
 	int exponent = field->unit_exponent;
 
 	pr_debug("time field exponent: %d\n", exponent);
-	for (; exponent < FF_TIME_EXPONENT; exponent++)
+	for (; scaled_time && exponent < FF_TIME_EXPONENT; exponent++)
 		scaled_time *= 10;
-	for (; exponent > FF_TIME_EXPONENT; exponent--)
+	for (; scaled_time && exponent > FF_TIME_EXPONENT; exponent--)
 		scaled_time /= 10;
 
 	pr_debug("time calculated from %d to %d\n", time, scaled_time);
@@ -1428,11 +1428,19 @@ static int pidff_find_effects(struct pidff_device *pidff,
  */
 static int pidff_init_fields(struct pidff_device *pidff, struct input_dev *dev)
 {
+	int exp;
+
 	if (PIDFF_FIND_FIELDS(set_effect, PID_SET_EFFECT, 1)) {
 		hid_err(pidff->hid, "unknown set_effect report layout\n");
 		return -ENODEV;
 	}
 
+	exp = pidff->set_effect[PID_DURATION].field->unit_exponent;
+	if (exp < -8 || exp > 0)
+		hid_warn(pidff->hid,
+			 "duration field has unusual time unit exponent %d\n",
+			 exp);
+
 	PIDFF_FIND_FIELDS(block_load, PID_BLOCK_LOAD, 0);
 	if (!pidff->block_load[PID_EFFECT_BLOCK_INDEX].value) {
 		hid_err(pidff->hid, "unknown pid_block_load report layout\n");

---
base-commit: d135ab453a7400a34930bdda1ef4bdc8acfac80e
change-id: 20261028-b4-pidff-rescale-exponent-1d5c4e218e6d

Best regards,
--  
Shengzhuo Wei <me@cherr.cc>