mm/damon/sysfs: fix use-after-free in state_show()

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

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 also occur with damon_sysfs_kdamonds_rm_dirs() and
damon_sysfs_kdamond_release(), which free or replace the context under
damon_sysfs_lock.)

Fix by taking damon_sysfs_lock before dereferencing the context, mirroring
the locking used in pid_show().

The bug has existed since state_show() first accessed kdamond->damon_ctx.

Link: https://lkml.kernel.org/r/20250905101046.2288-1-disclosure@aisle.com
Fixes: a61ea56 ("mm/damon/sysfs: link DAMON for virtual address spaces monitoring")
Signed-off-by: Stanislav Fort <disclosure@aisle.com>
Reported-by: Stanislav Fort <disclosure@aisle.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: stanislavfortaisle

mm/damon/sysfs.c

@@ -1260,14 +1260,18 @@ static ssize_t state_show(struct kobject *kobj, struct kobj_attribute *attr,
 {
 	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 (!ctx)
-		running = false;
-	else
+	if (!mutex_trylock(&damon_sysfs_lock))
+		return -EBUSY;
+
+	ctx = kdamond->damon_ctx;
+	if (ctx)
 		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]);