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Date:   Mon, 15 Aug 2022 02:46:36 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Christian Brauner <brauner@kernel.org>,
        Dongliang Mu <mudongliangabcd@gmail.com>,
        Dongliang Mu <dzm91@hust.edu.cn>,
        Arve =?iso-8859-1?B?SGr4bm5lduVn?= <arve@android.com>,
        Todd Kjos <tkjos@android.com>,
        Martijn Coenen <maco@android.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Carlos Llamas <cmllamas@google.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Kees Cook <keescook@chromium.org>,
        syzkaller <syzkaller@googlegroups.com>,
        linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] drivers: binderfs: fix memory leak in binderfs_fill_super
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On Fri, Aug 12, 2022 at 04:32:28PM +0200, Greg Kroah-Hartman wrote:

> > It's a bit tricky to follow but d_make_root() always consumes the inode.
> > On success via d_instantiate() and on failure via iput(). So when
> > d_make_root() has been called the inode is off limits. And as soon as
> > d_make_root() has returned successfully we're guaranteed that
> > sb->s_fs_info is cleaned up if a ->put_super() method has been defined.
> > Just fyi.
> 
> Ah, thanks, that wasn't obvious at all.
> 
> greg k-h

FWIW, I would rather provide a proper ->kill_sb() and gotten rid of
all that stuff.  The thing is, unlike ->put_super(), ->kill_sb() is
called for *anything* that has gotten to foo_fill_super().  Usually
allows to get rid of those "call all of or parts of foo_put_super()
on failure exits" and associated bitrot...

Like this (completely untested):

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
diff --git a/drivers/android/binderfs.c b/drivers/android/binderfs.c
index 588d753a7a19..c760f3129768 100644
--- a/drivers/android/binderfs.c
+++ b/drivers/android/binderfs.c
@@ -340,22 +340,21 @@ static int binderfs_show_options(struct seq_file *seq, struct dentry *root)
 	return 0;
 }
 
-static void binderfs_put_super(struct super_block *sb)
+static void binderfs_kill_super(struct super_block *sb)
 {
 	struct binderfs_info *info = sb->s_fs_info;
 
+	kill_litter_super(sb);
 	if (info && info->ipc_ns)
 		put_ipc_ns(info->ipc_ns);
 
 	kfree(info);
-	sb->s_fs_info = NULL;
 }
 
 static const struct super_operations binderfs_super_ops = {
 	.evict_inode    = binderfs_evict_inode,
 	.show_options	= binderfs_show_options,
 	.statfs         = simple_statfs,
-	.put_super	= binderfs_put_super,
 };
 
 static inline bool is_binderfs_control_device(const struct dentry *dentry)
@@ -789,7 +788,7 @@ static struct file_system_type binder_fs_type = {
 	.name			= "binder",
 	.init_fs_context	= binderfs_init_fs_context,
 	.parameters		= binderfs_fs_parameters,
-	.kill_sb		= kill_litter_super,
+	.kill_sb		= binderfs_kill_super,
 	.fs_flags		= FS_USERNS_MOUNT,
 };