[RFC PATCH 2/2] mm/damon/sysfs-schemes: protect path kfree() with damon_sysfs_lock

[SeongJae Park <sj@kernel.org>]

damon_sysfs_quot_goal->path can be read and written by users, via DAMON
sysfs memcg_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.

Fixes: c41e253a411e ("mm/damon/sysfs-schemes: implement path file under quota goal directory")
Cc: <stable@vger.kernel.org> # 6.19.x
Co-developed-by: Junxi Qian <qjx1298677004@gmail.com>
Signed-off-by: Junxi Qian <qjx1298677004@gmail.com>
Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/sysfs-schemes.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/mm/damon/sysfs-schemes.c b/mm/damon/sysfs-schemes.c
index 8d32a20531d49..245d63808411a 100644
--- a/mm/damon/sysfs-schemes.c
+++ b/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;
 }
 
-- 
2.47.3