mm/damon/stat: deallocate damon_call() failure leaking damon_ctx

damon_stat_start() always allocates the module's damon_ctx object
(damon_stat_context).  Meanwhile, if damon_call() in the function fails,
the damon_ctx object is not deallocated.  Hence, if the damon_call() is
failed, and the user writes Y to “enabled” again, the previously
allocated damon_ctx object is leaked.

This cannot simply be fixed by deallocating the damon_ctx object when
damon_call() fails.  That's because damon_call() failure doesn't guarantee
the kdamond main function, which accesses the damon_ctx object, is
completely finished.  In other words, if damon_stat_start() deallocates
the damon_ctx object after damon_call() failure, the not-yet-terminated
kdamond could access the freed memory (use-after-free).

Fix the leak while avoiding the use-after-free by keeping returning
damon_stat_start() without deallocating the damon_ctx object after
damon_call() failure, but deallocating it when the function is invoked
again and the kdamond is completely terminated.  If the kdamond is not yet
terminated, simply return -EAGAIN, as the kdamond will soon be terminated.

The issue was discovered [1] by sashiko.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lore.kernel.org/[email protected] [1]
Fixes: 405f619 ("mm/damon/stat: use damon_call() repeat mode instead of damon_callback")
Signed-off-by: SeongJae Park <[email protected]>
Cc: <[email protected]> # 6.17.x
Signed-off-by: Andrew Morton <[email protected]>

Author: sjp38

mm/damon/stat.c

@@ -245,6 +245,12 @@ static int damon_stat_start(void)
 {
 	int err;
 
+	if (damon_stat_context) {
+		if (damon_is_running(damon_stat_context))
+			return -EAGAIN;
+		damon_destroy_ctx(damon_stat_context);
+	}
+
 	damon_stat_context = damon_stat_build_ctx();
 	if (!damon_stat_context)
 		return -ENOMEM;
@@ -261,6 +267,7 @@ static void damon_stat_stop(void)
 {
 	damon_stop(&damon_stat_context, 1);
 	damon_destroy_ctx(damon_stat_context);
+	damon_stat_context = NULL;
 }
 
 static int damon_stat_enabled_store(