The cleanup tasks of kdamond threads including reset of corresponding DAMON context's ->kdamond field and decrease of global nr_running_ctxs counter is supposed to be executed by kdamond_fn(). However, commit 0f91d13366a4 ("mm/damon: simplify stop mechanism") made neither damon_start() nor damon_stop() ensure the corresponding kdamond has started the execution of kdamond_fn().
As a result, the cleanup can be skipped if damon_stop() is called fast enough after the previous damon_start(). Especially the skipped reset of ->kdamond could cause a use-after-free.
Fix it by waiting for start of kdamond_fn() execution from damon_start().
Link: https://lkml.kernel.org/r/20231208175018.63880-1-sj@kernel.org Fixes: 0f91d13366a4 ("mm/damon: simplify stop mechanism") Signed-off-by: SeongJae Park sj@kernel.org Reported-by: Jakub Acs acsjakub@amazon.de Cc: Changbin Du changbin.du@intel.com Cc: Jakub Acs acsjakub@amazon.de Cc: stable@vger.kernel.org # 5.15.x Signed-off-by: Andrew Morton akpm@linux-foundation.org (cherry picked from commit 6376a824595607e99d032a39ba3394988b4fce96) --- include/linux/damon.h | 2 ++ mm/damon/core.c | 7 +++++++ 2 files changed, 9 insertions(+)
diff --git a/include/linux/damon.h b/include/linux/damon.h index c70cca8a839f..e9e8ed92567a 100644 --- a/include/linux/damon.h +++ b/include/linux/damon.h @@ -524,6 +524,8 @@ struct damon_ctx { /* private: internal use only */ struct timespec64 last_aggregation; struct timespec64 last_ops_update; + /* for waiting until the execution of the kdamond_fn is started */ + struct completion kdamond_started;
/* public: */ struct task_struct *kdamond; diff --git a/mm/damon/core.c b/mm/damon/core.c index fd5be73f699f..191bc87b2441 100644 --- a/mm/damon/core.c +++ b/mm/damon/core.c @@ -423,6 +423,8 @@ struct damon_ctx *damon_new_ctx(void) if (!ctx) return NULL;
+ init_completion(&ctx->kdamond_started); + ctx->attrs.sample_interval = 5 * 1000; ctx->attrs.aggr_interval = 100 * 1000; ctx->attrs.ops_update_interval = 60 * 1000 * 1000; @@ -626,11 +628,14 @@ static int __damon_start(struct damon_ctx *ctx) mutex_lock(&ctx->kdamond_lock); if (!ctx->kdamond) { err = 0; + reinit_completion(&ctx->kdamond_started); ctx->kdamond = kthread_run(kdamond_fn, ctx, "kdamond.%d", nr_running_ctxs); if (IS_ERR(ctx->kdamond)) { err = PTR_ERR(ctx->kdamond); ctx->kdamond = NULL; + } else { + wait_for_completion(&ctx->kdamond_started); } } mutex_unlock(&ctx->kdamond_lock); @@ -1370,6 +1375,8 @@ static int kdamond_fn(void *data)
pr_debug("kdamond (%d) starts\n", current->pid);
+ complete(&ctx->kdamond_started); + if (ctx->ops.init) ctx->ops.init(ctx); if (ctx->callback.before_start && ctx->callback.before_start(ctx))