linux-kernel - [PATCH] mm/damon/sysfs: fix use-after-free in state

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Message-Id: <20250904175549.88928-1-disclosure@aisle.com>
Date: Thu,  4 Sep 2025 20:55:49 +0300
From: Stanislav Fort <stanislav.fort@...le.com>
To: SeongJae Park <sj@...nel.org>
Cc: damon@...ts.linux.dev,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Stanislav Fort <disclosure@...le.com>
Subject: [PATCH] mm/damon/sysfs: fix use-after-free in state_show()

state_show() currently reads kdamond->damon_ctx without holding
damon_sysfs_lock. This creates a use-after-free race condition:

CPU 0                           CPU 1
-----                           -----
state_show()                    damon_sysfs_turn_damon_on()
  ctx = kdamond->damon_ctx;
                                mutex_lock(&damon_sysfs_lock);
                                damon_destroy_ctx(kdamond->damon_ctx);
                                kdamond->damon_ctx = NULL;
                                mutex_unlock(&damon_sysfs_lock);
  damon_is_running(ctx);        /* ctx is freed */
    mutex_lock(&ctx->kdamond_lock);  /* UAF */

The race can occur with other functions that free or replace the context
while holding damon_sysfs_lock, such as damon_sysfs_kdamonds_rm_dirs()
and damon_sysfs_kdamond_release().

Fix this by acquiring damon_sysfs_lock before accessing the context,
mirroring the locking pattern used in pid_show().

This vulnerability was present when state_show() was first introduced to
access kdamond->damon_ctx.

Fixes: a61ea561c871 ("mm/damon/sysfs: link DAMON for virtual address spaces monitoring")
Reported-by: Stanislav Fort <disclosure@...le.com>
Signed-off-by: Stanislav Fort <disclosure@...le.com>
---
 mm/damon/sysfs.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index 1234567..abcdef0 100644
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -1258,17 +1258,24 @@ static ssize_t state_show(struct kobject *kobj, struct kobj_attribute *attr,
 		char *buf)
 {
 	struct damon_sysfs_kdamond *kdamond = container_of(kobj,
 			struct damon_sysfs_kdamond, kobj);
-	struct damon_ctx *ctx = kdamond->damon_ctx;
-	bool running;
+	struct damon_ctx *ctx;
+	bool running = false;
+
+	if (!mutex_trylock(&damon_sysfs_lock))
+		return -EBUSY;
+
+	ctx = kdamond->damon_ctx;
+	if (ctx)
+		running = damon_is_running(ctx);
 
-	if (!ctx)
-		running = false;
-	else
-		running = damon_is_running(ctx);
+	mutex_unlock(&damon_sysfs_lock);
 
 	return sysfs_emit(buf, "%s\n", running ?
 			damon_sysfs_cmd_strs[DAMON_SYSFS_CMD_ON] :
 			damon_sysfs_cmd_strs[DAMON_SYSFS_CMD_OFF]);
 }
 
 static int damon_sysfs_set_attrs(struct damon_ctx *ctx,
-- 
2.34.1