HID: bpf: prevent buffer overflow in hid_hw_request

right now the returned value is considered to be always valid. However,
when playing with HID-BPF, the return value can be arbitrary big,
because it's the return value of dispatch_hid_bpf_raw_requests(), which
calls the struct_ops and we have no guarantees that the value makes
sense.

Fixes: 8bd0488b5e ("HID: bpf: add HID-BPF hooks for hid_hw_raw_requests")
Cc: [email protected]
Acked-by: Jiri Kosina <[email protected]>
Signed-off-by: Benjamin Tissoires <[email protected]>

drivers/hid/bpf/hid_bpf_dispatch.c

@@ -444,6 +444,8 @@ hid_bpf_hw_request(struct hid_bpf_ctx *ctx, __u8 *buf, size_t buf__sz,
 					      (u64)(long)ctx,
 					      true); /* prevent infinite recursions */
 
+	if (ret > size)
+		ret = size;
 	if (ret > 0)
 		memcpy(buf, dma_data, ret);