kunit: fix UAF when run kfence test case test_gfpzero

Patch series "kunit: fix a UAF bug and do some optimization", v2.

This series is to fix UAF (use after free) when running kfence test case
test_gfpzero, which is time costly.  This UAF bug can be easily triggered
by setting CONFIG_KFENCE_NUM_OBJECTS = 65535.  Furthermore, some
optimization for kunit tests has been done.

This patch (of 3):

Kunit will create a new thread to run an actual test case, and the main
process will wait for the completion of the actual test thread until
overtime.  The variable "struct kunit test" has local property in function
kunit_try_catch_run, and will be used in the test case thread.  Task
kunit_try_catch_run will free "struct kunit test" when kunit runs
overtime, but the actual test case is still run and an UAF bug will be
triggered.

The above problem has been both observed in a physical machine and qemu
platform when running kfence kunit tests.  The problem can be triggered
when setting CONFIG_KFENCE_NUM_OBJECTS = 65535.  Under this setting, the
test case test_gfpzero will cost hours and kunit will run to overtime.
The follows show the panic log.

  BUG: unable to handle page fault for address: ffffffff82d882e9

  Call Trace:
   kunit_log_append+0x58/0xd0
   ...
   test_alloc.constprop.0.cold+0x6b/0x8a [kfence_test]
   test_gfpzero.cold+0x61/0x8ab [kfence_test]
   kunit_try_run_case+0x4c/0x70
   kunit_generic_run_threadfn_adapter+0x11/0x20
   kthread+0x166/0x190
   ret_from_fork+0x22/0x30
  Kernel panic - not syncing: Fatal exception
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
  Ubuntu-1.8.2-1ubuntu1 04/01/2014

To solve this problem, the test case thread should be stopped when the
kunit frame runs overtime.  The stop signal will send in function
kunit_try_catch_run, and test_gfpzero will handle it.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Peng Liu <[email protected]>
Reviewed-by: Marco Elver <[email protected]>
Reviewed-by: Brendan Higgins <[email protected]>
Tested-by: Brendan Higgins <[email protected]>
Cc: Alexander Potapenko <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Wang Kefeng <[email protected]>
Cc: Daniel Latypov <[email protected]>
Cc: David Gow <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

lib/kunit/try-catch.c

@@ -78,6 +78,7 @@ void kunit_try_catch_run(struct kunit_try_catch *try_catch, void *context)
 	if (time_remaining == 0) {
 		kunit_err(test, "try timed out\n");
 		try_catch->try_result = -ETIMEDOUT;
+		kthread_stop(task_struct);
 	}
 
 	exit_code = try_catch->try_result;

mm/kfence/kfence_test.c

@@ -623,7 +623,7 @@ static void test_gfpzero(struct kunit *test)
 			break;
 		test_free(buf2);
 
-		if (i == CONFIG_KFENCE_NUM_OBJECTS) {
+		if (kthread_should_stop() || (i == CONFIG_KFENCE_NUM_OBJECTS)) {
 			kunit_warn(test, "giving up ... cannot get same object back\n");
 			return;
 		}