[PATCH v2] mm/damon/sysfs-schemes: fix use-after-free on memcg_path and goal path

[Junxi Qian <qjx1298677004@gmail.com>]

memcg_path_store() and path_store() free and replace their respective
string pointers without holding damon_sysfs_lock. This creates a race
with the kdamond thread, which reads these pointers in
damon_sysfs_add_scheme_filters() and damos_sysfs_add_quota_score()
via damon_call() during a "commit" operation.

The race window is as follows:
  Thread A (commit):  holds damon_sysfs_lock, triggers damon_call()
  kdamond thread:     reads sysfs_filter->memcg_path in
                      damon_sysfs_memcg_path_to_id()
  Thread B (sysfs):   calls memcg_path_store() WITHOUT lock,
                      kfree(filter->memcg_path), replaces pointer

Since Thread B does not hold damon_sysfs_lock, the kdamond thread can
observe a freed pointer, resulting in a use-after-free when
damon_sysfs_memcg_path_eq() dereferences it for string comparison.

Similarly, memcg_path_show() and path_show() read the string pointers
without holding the lock, so a concurrent store can free the string
just before sysfs_emit() dereferences it, causing a use-after-free.

KASAN reports the following on v7.0.0-rc5 with CONFIG_KASAN=y:

  ==================================================================
  BUG: KASAN: double-free in memcg_path_store+0x6e/0xc0

  Free of addr ffff888100a086c0 by task exp/149

  CPU: 1 UID: 0 PID: 149 Comm: exp Not tainted 7.0.0-rc5-gd38efd7c139a #18 PREEMPT(lazy)
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
  Call Trace:
   <TASK>
   dump_stack_lvl+0x55/0x70
   print_report+0xcb/0x5d0
   kasan_report_invalid_free+0x94/0xc0
   check_slab_allocation+0xde/0x110
   kfree+0x114/0x3b0
   memcg_path_store+0x6e/0xc0
   kernfs_fop_write_iter+0x2fc/0x490
   vfs_write+0x8e1/0xcc0
   ksys_write+0xee/0x1c0
   do_syscall_64+0xfc/0x580
   entry_SYSCALL_64_after_hwframe+0x77/0x7f
   </TASK>

  Allocated by task 147 on cpu 0 at 12.364295s:
   kasan_save_stack+0x24/0x50
   kasan_save_track+0x17/0x60
   __kasan_kmalloc+0x7f/0x90
   __kmalloc_noprof+0x191/0x490
   memcg_path_store+0x32/0xc0
   kernfs_fop_write_iter+0x2fc/0x490
   vfs_write+0x8e1/0xcc0
   ksys_write+0xee/0x1c0

  Freed by task 150 on cpu 0 at 13.373810s:
   kasan_save_stack+0x24/0x50
   kasan_save_track+0x17/0x60
   kasan_save_free_info+0x3b/0x60
   __kasan_slab_free+0x43/0x70
   kfree+0x137/0x3b0
   memcg_path_store+0x6e/0xc0
   kernfs_fop_write_iter+0x2fc/0x490
   vfs_write+0x8e1/0xcc0
   ksys_write+0xee/0x1c0

  The buggy address belongs to the object at ffff888100a086c0
   which belongs to the cache kmalloc-8 of size 8
  ==================================================================

Fix this by holding damon_sysfs_lock while swapping the path pointer
in both memcg_path_store() and path_store(), and while reading the
path pointer in memcg_path_show() and path_show(). The actual kfree()
is moved outside the lock since the old pointer is no longer reachable
once replaced.

Fixes: 490a43d07f16 ("mm/damon/sysfs-schemes: free old damon_sysfs_scheme_filter->memcg_path on write")
Signed-off-by: Junxi Qian <qjx1298677004@gmail.com>

---
Changes in v2:
- Also protect memcg_path_show() and path_show() with damon_sysfs_lock,
  since sysfs show and store callbacks can run concurrently and a
  lockless read in show can race with the kfree in store. (Sashiko AI)

 mm/damon/sysfs-schemes.c | 36 ++++++++++++++++++++++++++++--------
 1 file changed, 28 insertions(+), 8 deletions(-)

diff --git a/mm/damon/sysfs-schemes.c b/mm/damon/sysfs-schemes.c
index 5186966da..1a890e2a4 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);
+	ssize_t len;
 
-	return sysfs_emit(buf, "%s\n",
+	mutex_lock(&damon_sysfs_lock);
+	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,
@@ -543,15 +548,20 @@ static ssize_t memcg_path_store(struct kobject *kobj,
 {
 	struct damon_sysfs_scheme_filter *filter = container_of(kobj,
 			struct damon_sysfs_scheme_filter, kobj);
-	char *path = kmalloc_array(size_add(count, 1), sizeof(*path),
-				   GFP_KERNEL);
+	char *path, *old;
 
+	path = kmalloc_array(size_add(count, 1), sizeof(*path), GFP_KERNEL);
 	if (!path)
 		return -ENOMEM;
 
 	strscpy(path, buf, count + 1);
-	kfree(filter->memcg_path);
+
+	mutex_lock(&damon_sysfs_lock);
+	old = filter->memcg_path;
 	filter->memcg_path = path;
+	mutex_unlock(&damon_sysfs_lock);
+
+	kfree(old);
 	return count;
 }
 
@@ -1187,8 +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);
+	ssize_t len;
 
-	return sysfs_emit(buf, "%s\n", goal->path ? goal->path : "");
+	mutex_lock(&damon_sysfs_lock);
+	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,
@@ -1196,15 +1211,20 @@ static ssize_t path_store(struct kobject *kobj,
 {
 	struct damos_sysfs_quota_goal *goal = container_of(kobj,
 			struct damos_sysfs_quota_goal, kobj);
-	char *path = kmalloc_array(size_add(count, 1), sizeof(*path),
-				   GFP_KERNEL);
+	char *path, *old;
 
+	path = kmalloc_array(size_add(count, 1), sizeof(*path), GFP_KERNEL);
 	if (!path)
 		return -ENOMEM;
 
 	strscpy(path, buf, count + 1);
-	kfree(goal->path);
+
+	mutex_lock(&damon_sysfs_lock);
+	old = goal->path;
 	goal->path = path;
+	mutex_unlock(&damon_sysfs_lock);
+
+	kfree(old);
 	return count;
 }
 
-- 
2.34.1