modules: only allow symbol_get of EXPORT_SYMBOL_GPL modules

It has recently come to my attention that nvidia is circumventing the
protection added in 262e6ae ("modules: inherit
TAINT_PROPRIETARY_MODULE") by importing exports from their proprietary
modules into an allegedly GPL licensed module and then rexporting them.

Given that symbol_get was only ever intended for tightly cooperating
modules using very internal symbols it is logical to restrict it to
being used on EXPORT_SYMBOL_GPL and prevent nvidia from costly DMCA
Circumvention of Access Controls law suites.

All symbols except for four used through symbol_get were already exported
as EXPORT_SYMBOL_GPL, and the remaining four ones were switched over in
the preparation patches.

Fixes: 262e6ae ("modules: inherit TAINT_PROPRIETARY_MODULE")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>

Author: Christoph Hellwig

kernel/module/main.c

@@ -1295,12 +1295,20 @@ void *__symbol_get(const char *symbol)
 	};
 
 	preempt_disable();
-	if (!find_symbol(&fsa) || strong_try_module_get(fsa.owner)) {
-		preempt_enable();
-		return NULL;
+	if (!find_symbol(&fsa))
+		goto fail;
+	if (fsa.license != GPL_ONLY) {
+		pr_warn("failing symbol_get of non-GPLONLY symbol %s.\n",
+			symbol);
+		goto fail;
 	}
+	if (strong_try_module_get(fsa.owner))
+		goto fail;
 	preempt_enable();
 	return (void *)kernel_symbol_value(fsa.sym);
+fail:
+	preempt_enable();
+	return NULL;
 }
 EXPORT_SYMBOL_GPL(__symbol_get);