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

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

[Shengzhuo Wei]

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.

Fixes: 8713107221a8 ("HID: pidff: Rescale time values to match field units")
Cc: stable@vger.kernel.org
Signed-off-by: Shengzhuo Wei <me@cherr.cc>
---
 drivers/hid/usbhid/hid-pidff.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/hid/usbhid/hid-pidff.c b/drivers/hid/usbhid/hid-pidff.c
index 22951b7ecd17..1a2fffcf204e 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);

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

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

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

[sashiko-bot]

> 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.
> 
> Fixes: 8713107221a8 ("HID: pidff: Rescale time values to match field units")
> Cc: stable@vger.kernel.org
> Signed-off-by: Shengzhuo Wei <me@cherr.cc>

Sashiko has reviewed this patch and found no issues.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261029-b4-pidff-rescale-exponent-v1-1-3ba54accf767@cherr.cc?part=1