[PATCH] HID: core: fix number/pointer type confusion on long items

[Jann Horn]

When fetch_item() is called by hid_scan_report() on an item with
HID_ITEM_TAG_LONG, it stores a pointer to the item data in
item->data.longdata instead of storing a value directly in
item->data.{u8/u16/u32}.

When item_udata() or item_sdata() encounters such an item, it incorrectly
assumes that the item is in short format, and therefore returns the lower
part of a kernel pointer reinterpreted as a number.

When a HID device is connected whose descriptor contains a
HID_GLOBAL_ITEM_TAG_REPORT_SIZE encoded in long format with size=4, this
causes the lower half of a kernel pointer to be printed into dmesg as a
number, like this:

    hid (null): invalid report_size 107953555

To fix it, let item_udata() and item_sdata() verify that the item is in
short format.

Note that this bug only affects hid_scan_report(), while the main parsing
pass hid_parse_collections() will always bail out when encountering a long
item.

Sidenote: There are currently no users of data.longdata; maybe we should
just remove any parsing of long-format descriptors as a follow-up.

Fixes: 3dc8fc083dbf ("HID: Use hid_parser for pre-scanning the report descriptors")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
 drivers/hid/hid-core.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/hid/hid-core.c b/drivers/hid/hid-core.c
index 41a79e43c82b..d6676505e122 100644
--- a/drivers/hid/hid-core.c
+++ b/drivers/hid/hid-core.c
@@ -379,6 +379,9 @@ static int hid_add_field(struct hid_parser *parser, unsigned report_type, unsign
 
 static u32 item_udata(struct hid_item *item)
 {
+	if (item->format != HID_ITEM_FORMAT_SHORT)
+		return 0;
+
 	switch (item->size) {
 	case 1: return item->data.u8;
 	case 2: return item->data.u16;
@@ -389,6 +392,9 @@ static u32 item_udata(struct hid_item *item)
 
 static s32 item_sdata(struct hid_item *item)
 {
+	if (item->format != HID_ITEM_FORMAT_SHORT)
+		return 0;
+
 	switch (item->size) {
 	case 1: return item->data.s8;
 	case 2: return item->data.s16;

---
base-commit: 51512e22efe813d8223de27f6fd02a8a48ea2323
change-id: 20260703-hid-core-data-typeconfusion-95c660affffe

Best regards,
--  
Jann Horn <jannh@xxxxxxxxxx>