mm/damon/sysfs-schemes: protect path kfree() with damon_sysfs_lock

commit cf3b714 upstream.

damon_sysfs_quot_goal->path can be read and written by users, via DAMON
sysfs 'path' file.  It can also be indirectly read, for the parameters
{on,off}line committing to DAMON.  The reads for parameters committing are
protected by damon_sysfs_lock to avoid the sysfs files being destroyed
while any of the parameters are being read.  But the user-driven direct
reads and writes are not protected by any lock, while the write is
deallocating the path-pointing buffer.  As a result, the readers could
read the already freed buffer (user-after-free).  Note that the user-reads
don't race when the same open file is used by the writer, due to kernfs's
open file locking.  Nonetheless, doing the reads and writes with separate
open files would be common.  Fix it by protecting both the user-direct
reads and writes with damon_sysfs_lock.

Link: https://lore.kernel.org/20260423150253.111520-3-sj@kernel.org
Fixes: c41e253 ("mm/damon/sysfs-schemes: implement path file under quota goal directory")
Co-developed-by: Junxi Qian <qjx1298677004@gmail.com>
Signed-off-by: Junxi Qian <qjx1298677004@gmail.com>
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: <stable@vger.kernel.org> # 6.19.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: sjp38

mm/damon/sysfs-schemes.c

@@ -1197,8 +1197,13 @@ static ssize_t path_show(struct kobject *kobj,
 {
 	struct damos_sysfs_quota_goal *goal = container_of(kobj,
 			struct damos_sysfs_quota_goal, kobj);
+	int len;
 
-	return sysfs_emit(buf, "%s\n", goal->path ? goal->path : "");
+	if (!mutex_trylock(&damon_sysfs_lock))
+		return -EBUSY;
+	len = sysfs_emit(buf, "%s\n", goal->path ? goal->path : "");
+	mutex_unlock(&damon_sysfs_lock);
+	return len;
 }
 
 static ssize_t path_store(struct kobject *kobj,
@@ -1213,8 +1218,13 @@ static ssize_t path_store(struct kobject *kobj,
 		return -ENOMEM;
 
 	strscpy(path, buf, count + 1);
+	if (!mutex_trylock(&damon_sysfs_lock)) {
+		kfree(path);
+		return -EBUSY;
+	}
 	kfree(goal->path);
 	goal->path = path;
+	mutex_unlock(&damon_sysfs_lock);
 	return count;
 }