kunit: Device wrappers should also manage driver name

kunit_driver_create() accepts a name for the driver, but does not copy
it, so if that name is either on the stack, or otherwise freed, we end
up with a use-after-free when the driver is cleaned up.

Instead, strdup() the name, and manage it as another KUnit allocation.
As there was no existing kunit_kstrdup(), we add one. Further, add a
kunit_ variant of strdup_const() and kfree_const(), so we don't need to
allocate and manage the string in the majority of cases where it's a
constant.

However, these are inline functions, and is_kernel_rodata() only works
for built-in code. This causes problems in two cases:
- If kunit is built as a module, __{start,end}_rodata is not defined.
- If a kunit test using these functions is built as a module, it will
suffer the same fate.

This fixes a KASAN splat with overflow.overflow_allocation_test, when
built as a module.

Restrict the is_kernel_rodata() case to when KUnit is built as a module,
which fixes the first case, at the cost of losing the optimisation.

Also, make kunit_{kstrdup,kfree}_const non-inline, so that other modules
using them will not accidentally depend on is_kernel_rodata(). If KUnit
is built-in, they'll benefit from the optimisation, if KUnit is not,
they won't, but the string will be properly duplicated.

Fixes: d03c720e03bd ("kunit: Add APIs for managing devices")
Reported-by: Nico Pache <[e-mail]>
Closes: https://groups.google.com/g/kunit-dev/c/81V9b9QYON0
Reviewed-by: Kees Cook <[e-mail]>
Reviewed-by: Maxime Ripard <[e-mail]>
Reviewed-by: Rae Moar <[e-mail]>
Signed-off-by: David Gow <[e-mail]>
Tested-by: Rae Moar <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

include/kunit/test.h

···
 #include <linux/types.h>
 
 #include <asm/rwonce.h>
+#include <asm/sections.h>
 
 /* Static key: true if any KUnit tests are currently running */
 DECLARE_STATIC_KEY_FALSE(kunit_running);
···
 {
 	return kunit_kmalloc_array(test, n, size, gfp | __GFP_ZERO);
 }
+
+
+/**
+ * kunit_kfree_const() - conditionally free test managed memory
+ * @x: pointer to the memory
+ *
+ * Calls kunit_kfree() only if @x is not in .rodata section.
+ * See kunit_kstrdup_const() for more information.
+ */
+void kunit_kfree_const(struct kunit *test, const void *x);
+
+/**
+ * kunit_kstrdup() - Duplicates a string into a test managed allocation.
+ *
+ * @test: The test context object.
+ * @str: The NULL-terminated string to duplicate.
+ * @gfp: flags passed to underlying kmalloc().
+ *
+ * See kstrdup() and kunit_kmalloc_array() for more information.
+ */
+static inline char *kunit_kstrdup(struct kunit *test, const char *str, gfp_t gfp)
+{
+	size_t len;
+	char *buf;
+
+	if (!str)
+		return NULL;
+
+	len = strlen(str) + 1;
+	buf = kunit_kmalloc(test, len, gfp);
+	if (buf)
+		memcpy(buf, str, len);
+	return buf;
+}
+
+/**
+ * kunit_kstrdup_const() - Conditionally duplicates a string into a test managed allocation.
+ *
+ * @test: The test context object.
+ * @str: The NULL-terminated string to duplicate.
+ * @gfp: flags passed to underlying kmalloc().
+ *
+ * Calls kunit_kstrdup() only if @str is not in the rodata section. Must be freed with
+ * kunit_kfree_const() -- not kunit_kfree().
+ * See kstrdup_const() and kunit_kmalloc_array() for more information.
+ */
+const char *kunit_kstrdup_const(struct kunit *test, const char *str, gfp_t gfp);
 
 /**
  * kunit_vm_mmap() - Allocate KUnit-tracked vm_mmap() area

lib/kunit/device.c

···
 	if (!driver)
 		return ERR_PTR(err);
 
-	driver->name = name;
+	driver->name = kunit_kstrdup_const(test, name, GFP_KERNEL);
 	driver->bus = &kunit_bus_type;
 	driver->owner = THIS_MODULE;
 
···
 	const struct device_driver *driver = to_kunit_device(dev)->driver;
 
 	kunit_release_action(test, device_unregister_wrapper, dev);
-	if (driver)
+	if (driver) {
+		const char *driver_name = driver->name;
 		kunit_release_action(test, driver_unregister_wrapper, (void *)driver);
+		kunit_kfree_const(test, driver_name);
+	}
 }
 EXPORT_SYMBOL_GPL(kunit_device_unregister);
 

lib/kunit/test.c

···
 }
 EXPORT_SYMBOL_GPL(kunit_kfree);
 
+void kunit_kfree_const(struct kunit *test, const void *x)
+{
+#if !IS_MODULE(CONFIG_KUNIT)
+	if (!is_kernel_rodata((unsigned long)x))
+#endif
+		kunit_kfree(test, x);
+}
+EXPORT_SYMBOL_GPL(kunit_kfree_const);
+
+const char *kunit_kstrdup_const(struct kunit *test, const char *str, gfp_t gfp)
+{
+#if !IS_MODULE(CONFIG_KUNIT)
+	if (is_kernel_rodata((unsigned long)str))
+		return str;
+#endif
+	return kunit_kstrdup(test, str, gfp);
+}
+EXPORT_SYMBOL_GPL(kunit_kstrdup_const);
+
 void kunit_cleanup(struct kunit *test)
 {
 	struct kunit_resource *res;