[Linux-kernel-mentees] [PATCH v1] usbhid: Fix slab-out-of-bounds write in hiddev_ioctl_usage()

`uref->usage_index` is not always being properly checked, causing
hiddev_ioctl_usage() to go out of bounds under some cases. Fix it.

This patch fixes the following syzbot bug:

    https://syzkaller.appspot.com/bug?id=f2aebe90b8c56806b050a20b36f51ed6acabe802

Reported-by: syzbot+34ee1b45d88571c2fa8b@syzkaller.appspotmail.com
Signed-off-by: Peilin Ye <redacted>
---
This patch fixes the bug, but in an ugly way. Checks on `uref` are already
being done in this code:

			if (cmd == HIDIOCGCOLLECTIONINDEX) {
				if (uref->usage_index >= field->maxusage)
					goto inval;
				uref->usage_index =
					array_index_nospec(uref->usage_index,
							   field->maxusage);
			} else if (uref->usage_index >= field->report_count)
				goto inval;

However it did not catch this bug since it's in an `else` bracket. Should
we move the above code out of the bracket? Would like to hear your opinion.
Thank you!

 drivers/hid/usbhid/hiddev.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/hid/usbhid/hiddev.c b/drivers/hid/usbhid/hiddev.c
index 4140dea693e9..c63d07caacef 100644
--- a/drivers/hid/usbhid/hiddev.c
+++ b/drivers/hid/usbhid/hiddev.c

@@ -525,6 +525,8 @@ static noinline int hiddev_ioctl_usage(struct hiddev *hiddev, unsigned int cmd,
 			goto goodreturn;
 
 		case HIDIOCSUSAGE:
+			if (uref->usage_index >= field->report_count)
+				goto inval;
 			field->value[uref->usage_index] = uref->value;
 			goto goodreturn;
 

-- 
2.25.1