mm/damon/core: make damon_start() waits until kdamond_fn() starts

commit 6376a824595607e99d032a39ba3394988b4fce96 upstream.

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>
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/include/linux/damon.h b/include/linux/damon.h
index b13be7ae2275ea5b541df770e7dbae36ab1c3e02..e6941b239f449d35732cadb6ef78b2ba8a9598b5 100644 (file)
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -8,6 +8,7 @@
 #ifndef _DAMON_H_
 #define _DAMON_H_
 
+#include <linux/completion.h>
 #include <linux/mutex.h>
 #include <linux/time64.h>
 #include <linux/types.h>
@@ -452,6 +453,8 @@ 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 36d098d06c5583db2d555e40b7075ea1760b1468..5db9bec8ae67cd5a922635a7b9dbe65ae685b46c 100644 (file)
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -383,6 +383,8 @@ 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;
@@ -519,11 +521,14 @@ 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);
@@ -1147,6 +1152,8 @@ 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))