Re: [PATCH] HID: core: clamp report_size in s32ton() to avoid undefined shift

[Jiri Kosina]

On Mon, 6 Apr 2026, Greg Kroah-Hartman wrote:

> s32ton() shifts by n-1 where n is the field's report_size, a value that
> comes directly from a HID device.  The HID parser bounds report_size
> only to <= 256, so a broken HID device can supply a report descriptor
> with a wide field that triggers shift exponents up to 256 on a 32-bit
> type when an output report is built via hid_output_field() or
> hid_set_field().
> 
> Commit ec61b41918587 ("HID: core: fix shift-out-of-bounds in
> hid_report_raw_event") added the same n > 32 clamp to the function
> snto32(), but s32ton() was never given the same fix as I guess syzbot
> hadn't figured out how to fuzz a device the same way.
> 
> Fix this up by just clamping the max value of n, just like snto32()
> does.
> 
> Cc: stable <stable@xxxxxxxxxx>
> Cc: Jiri Kosina <jikos@xxxxxxxxxx>
> Cc: Benjamin Tissoires <bentiss@xxxxxxxxxx>
> Cc: linux-input@xxxxxxxxxxxxxxx
> Assisted-by: gregkh_clanker_t1000
> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

Applied, thanks.

-- 
Jiri Kosina
SUSE Labs