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

commit 1e68eb96e8beb1abefd12dd22c5637795d8a877e upstream.

Patch series "mm/damon/sysfs-schemes: fix use-after-free for [memcg_]path".

Reads of 'memcg_path' and 'path' files in DAMON sysfs interface could race
with their writes, results in use-after-free.  Fix those.

This patch (of 2):

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.

Link: https://lore.kernel.org/20260423150253.111520-1-sj@kernel.org
Link: https://lore.kernel.org/20260423150253.111520-2-sj@kernel.org
Fixes: 4f489fe6afb3 ("mm/damon/sysfs-schemes: free old damon_sysfs_scheme_filter->memcg_path on write")
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> # 6.16.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/mm/damon/sysfs-schemes.c b/mm/damon/sysfs-schemes.c
index 3a0782e576fab71fe88b9417b9c722094aec904f..fbba0016972d3f5bdc7b6fd410c6d1f9736784a9 100644 (file)
--- 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;
 }