misc: fastrpc: restrict user apps from sending kernel RPC messages

Verify that user applications are not using the kernel RPC message
handle to restrict them from directly attaching to guest OS on the
remote subsystem. This is a port of CVE-2019-2308 fix.

Fixes: c68cfb7 ("misc: fastrpc: Add support for context Invoke method")
Cc: Srinivas Kandagatla <[email protected]>
Cc: Jonathan Marek <[email protected]>
Cc: [email protected]
Signed-off-by: Dmitry Baryshkov <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: lumag

drivers/misc/fastrpc.c

@@ -950,6 +950,11 @@ static int fastrpc_internal_invoke(struct fastrpc_user *fl,  u32 kernel,
 	if (!fl->cctx->rpdev)
 		return -EPIPE;
 
+	if (handle == FASTRPC_INIT_HANDLE && !kernel) {
+		dev_warn_ratelimited(fl->sctx->dev, "user app trying to send a kernel RPC message (%d)\n",  handle);
+		return -EPERM;
+	}
+
 	ctx = fastrpc_context_alloc(fl, kernel, sc, args);
 	if (IS_ERR(ctx))
 		return PTR_ERR(ctx);