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

[Greg Kroah-Hartman]

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>
---
 drivers/hid/hid-core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/hid/hid-core.c b/drivers/hid/hid-core.c
index 833df14ef68f..868c65684aa8 100644
--- a/drivers/hid/hid-core.c
+++ b/drivers/hid/hid-core.c
@@ -71,6 +71,9 @@ static u32 s32ton(__s32 value, unsigned int n)
 	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;
-- 
2.53.0