[PATCH 0/2] mm/damon/sysfs-schemes: fix use-after-free for [memcg_]path

[PATCH 0/2] mm/damon/sysfs-schemes: fix use-after-free for [memcg_]path

[SeongJae Park]

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

Changes from RFC
- rfc: https://lore.kernel.org/20260422144059.72000-1-sj@kernel.org
- Fix a typo in commit description.
- Drop RFC tag.
- Rebase to latest mm-hotfixes-unstable.
Changes from v2
- v2: https://lore.kernel.org/20260420125405.362137-1-qjx1298677004@gmail.com
- Split patch for individual fixes commits.
- Hand-off authorship to SJ, give Co-developed-by: to Junxi.
- Use mutex_trylock() instead of mutex_lock().
- Add RFC tag for Sashiko review round.
- Wordsmith commit messages.
Changes from v1
- v1: https://lore.kernel.org/20260420085332.178473-1-qjx1298677004@gmail.com
- Protect not only user-writes but also user-reads.

SeongJae Park (2):
  mm/damon/sysfs-schemes: protect memcg_path kfree() with
    damon_sysfs_lock
  mm/damon/sysfs-schemes: protect path kfree() with damon_sysfs_lock

 mm/damon/sysfs-schemes.c | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)


base-commit: 26232ccdfcc7029d92b633b01f5f22913ab72168
-- 
2.47.3

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

[SeongJae Park]

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

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

[SeongJae Park]

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.

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