[PATCH 6.18 127/198] HID: core: clamp report_size in s32ton() to avoid undefined shift

Greg Kroah-Hartman
2026-04-20T16:02:42Z

From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

commit 69c02ffde6ed4d535fa4e693a9e572729cad3d0d upstream.

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@kernel.org>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Benjamin Tissoires <bentiss@kernel.org>
Cc: linux-input@vger.kernel.org
Assisted-by: gregkh_clanker_t1000
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jiri Kosina <jkosina@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/hid/hid-core.c | 3 +++
1 file changed, 3 insertions(+)

--- a/drivers/hid/hid-core.c
+++ b/drivers/hid/hid-core.c
@@ -71,6 +71,9 @@ static u32 s32ton(__s32 value, unsigned
if (!value || !n)
return 0;

+ if (n > 32)
+ n = 32;
+
a = value >> (n - 1);
if (a && a != -1)
return value < 0 ? 1 << (n - 1) : (1 << (n - 1)) - 1;