[RFC PATCH 1/2] mm/damon/sysfs-schemes: protect memcg_path kfree() with damon_sysfs_lock

[SeongJae Park <sj@kernel.org>]

damon_sysfs_scheme_filter->mmecg_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 memcg_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: 4f489fe6afb3 ("mm/damon/sysfs-schemes: free old damon_sysfs_scheme_filter->memcg_path on write")
Cc: <stable@vger.kernel.org> # 6.16.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 5186966dafb35..8d32a20531d49 100644
--- a/mm/damon/sysfs-schemes.c
+++ b/mm/damon/sysfs-schemes.c
@@ -533,9 +533,14 @@ static ssize_t memcg_path_show(struct kobject *kobj,
 {
 	struct damon_sysfs_scheme_filter *filter = container_of(kobj,
 			struct damon_sysfs_scheme_filter, kobj);
+	int len;
 
-	return sysfs_emit(buf, "%s\n",
+	if (!mutex_trylock(&damon_sysfs_lock))
+		return -EBUSY;
+	len = sysfs_emit(buf, "%s\n",
 			filter->memcg_path ? filter->memcg_path : "");
+	mutex_unlock(&damon_sysfs_lock);
+	return len;
 }
 
 static ssize_t memcg_path_store(struct kobject *kobj,
@@ -550,8 +555,13 @@ static ssize_t memcg_path_store(struct kobject *kobj,
 		return -ENOMEM;
 
 	strscpy(path, buf, count + 1);
+	if (!mutex_trylock(&damon_sysfs_lock)) {
+		kfree(path);
+		return -EBUSY;
+	}
 	kfree(filter->memcg_path);
 	filter->memcg_path = path;
+	mutex_unlock(&damon_sysfs_lock);
 	return count;
 }
 
-- 
2.47.3