[PATCH v2] hfsplus: fix s_fs_info leak on mount setup failure

[Shardul Bankar <shardulsb08@gmail.com>]

Syzkaller reported a memory leak in hfsplus where s_fs_info (sbi) is
allocated in hfsplus_init_fs_context() but never freed if the mount
setup fails during setup_bdev_super().

In get_tree_bdev_flags(), if setup_bdev_super() fails, the superblock
is torn down via deactivate_locked_super(). Since this failure occurs
before fill_super() is called, the superblock's operations (sb->s_op)
are not yet set. Consequently, the standard ->put_super() callback
cannot be invoked, and the allocated s_fs_info remains leaked.

Fix this by implementing a custom ->kill_sb() handler,
hfsplus_kill_sb(), which explicitly frees s_fs_info using RCU
synchronization. This ensures cleanup happens regardless of whether
fill_super() succeeded or ->put_super() was called.

To support this new lifecycle:
1. In hfsplus_put_super(), remove the call_rcu() call. The actual freeing
   of s_fs_info is deferred to hfsplus_kill_sb().
2. In hfsplus_fill_super(), remove the explicit cleanup of sbi (both kfree
   and unload_nls) in the error path. The VFS will call ->kill_sb() on
   failure, so retaining these would result in double-frees or refcount
   underflows.
3. Implement hfsplus_kill_sb() to invoke kill_block_super() and then free
   s_fs_info via RCU.

Reported-by: syzbot+99f6ed51479b86ac4c41@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=99f6ed51479b86ac4c41
Signed-off-by: Shardul Bankar <shardul.b@mpiricsoftware.com>
---
v1:
 - tried to fix the leak in fs/super.c (VFS layer).
 - Link: https://lore.kernel.org/all/20260201082724.GC3183987@ZenIV/
v2:
 - abandons the VFS changes in favor of a driver-specific fix in
   hfsplus, implementing a custom ->kill_sb() to handle the cleanup
   lifecycle, as suggested by Al Viro.

 fs/hfsplus/super.c | 30 ++++++++++++++++++------------
 1 file changed, 18 insertions(+), 12 deletions(-)

diff --git a/fs/hfsplus/super.c b/fs/hfsplus/super.c
index aaffa9e060a0..cc80cd545a3e 100644
--- a/fs/hfsplus/super.c
+++ b/fs/hfsplus/super.c
@@ -311,14 +311,6 @@ void hfsplus_mark_mdb_dirty(struct super_block *sb)
 	spin_unlock(&sbi->work_lock);
 }
 
-static void delayed_free(struct rcu_head *p)
-{
-	struct hfsplus_sb_info *sbi = container_of(p, struct hfsplus_sb_info, rcu);
-
-	unload_nls(sbi->nls);
-	kfree(sbi);
-}
-
 static void hfsplus_put_super(struct super_block *sb)
 {
 	struct hfsplus_sb_info *sbi = HFSPLUS_SB(sb);
@@ -344,7 +336,6 @@ static void hfsplus_put_super(struct super_block *sb)
 	hfs_btree_close(sbi->ext_tree);
 	kfree(sbi->s_vhdr_buf);
 	kfree(sbi->s_backup_vhdr_buf);
-	call_rcu(&sbi->rcu, delayed_free);
 
 	hfs_dbg("finished\n");
 }
@@ -648,9 +639,7 @@ static int hfsplus_fill_super(struct super_block *sb, struct fs_context *fc)
 	kfree(sbi->s_vhdr_buf);
 	kfree(sbi->s_backup_vhdr_buf);
 out_unload_nls:
-	unload_nls(sbi->nls);
 	unload_nls(nls);
-	kfree(sbi);
 	return err;
 }
 
@@ -709,10 +698,27 @@ static int hfsplus_init_fs_context(struct fs_context *fc)
 	return 0;
 }
 
+static void delayed_free(struct rcu_head *p)
+{
+	struct hfsplus_sb_info *sbi = container_of(p, struct hfsplus_sb_info, rcu);
+
+	unload_nls(sbi->nls);
+	kfree(sbi);
+}
+
+static void hfsplus_kill_sb(struct super_block *sb)
+{
+	struct hfsplus_sb_info *sbi = sb->s_fs_info;
+
+	kill_block_super(sb);
+	if (sbi)
+		call_rcu(&sbi->rcu, delayed_free);
+}
+
 static struct file_system_type hfsplus_fs_type = {
 	.owner		= THIS_MODULE,
 	.name		= "hfsplus",
-	.kill_sb	= kill_block_super,
+	.kill_sb	= hfsplus_kill_sb,
 	.fs_flags	= FS_REQUIRES_DEV,
 	.init_fs_context = hfsplus_init_fs_context,
 };
-- 
2.34.1