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. [1] https://lore.kernel.org/20260401012428.86694-1-sj@kernel.org Fixes: 405f61996d9d ("mm/damon/stat: use damon_call() repeat mode instead of damon_callback") Cc: # 6.17.x Signed-off-by: SeongJae Park --- Changes from RFC (https://lore.kernel.org/20260402045928.71170-1-sj@kernel.org) - Drop RFC tag again. - sashiko didn't find any real issue on this version. Changes from v1 (https://lore.kernel.org/20260402010457.66860-1-sj@kernel.org) - Avoid sashiko-discovered use-after-free. - Add RFC tag. mm/damon/stat.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/mm/damon/stat.c b/mm/damon/stat.c index 5a742fc157e4..99ba346f9e32 100644 --- 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; @@ -264,6 +270,7 @@ 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( base-commit: 4fd04f750d79667937931314ed64c9d79b0d82ef -- 2.47.3