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/20260402134418.74121-1-sj@kernel.org
Link: https://lore.kernel.org/20260401012428.86694-1-sj@kernel.org
Fixes: 405f61996d9d ("mm/damon/stat: use damon_call() repeat mode instead of damon_callback")
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: <stable@vger.kernel.org> # 6.17.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/damon/stat.c b/mm/damon/stat.c
index cf2c5a541eeeaab349c728a01337adf0859ba108..60351a7194606309a932d61f9f025b0cc8fac276 100644 (file)
--- a/mm/damon/stat.c
+++ b/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(