linux-kernel - Re: [PATCH] binderfs: rework superblock destruction

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Date:   Wed, 17 Aug 2022 16:51:44 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Dongliang Mu <mudongliangabcd@...il.com>,
        Dongliang Mu <dzm91@...t.edu.cn>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Carlos Llamas <cmllamas@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        syzkaller <syzkaller@...glegroups.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] binderfs: rework superblock destruction

On Wed, Aug 17, 2022 at 03:19:13PM +0100, Al Viro wrote:
> On Wed, Aug 17, 2022 at 04:01:49PM +0200, Christian Brauner wrote:
> > On Wed, Aug 17, 2022 at 02:59:02PM +0100, Al Viro wrote:
> > > On Wed, Aug 17, 2022 at 03:03:06PM +0200, Christian Brauner wrote:
> > > 
> > > > +static void binderfs_kill_super(struct super_block *sb)
> > > > +{
> > > > +	struct binderfs_info *info = sb->s_fs_info;
> > > > +
> > > > +	if (info && info->ipc_ns)
> > > > +		put_ipc_ns(info->ipc_ns);
> > > > +
> > > > +	kfree(info);
> > > > +	kill_litter_super(sb);
> > > > +}
> > > 
> > > Other way round, please - shut the superblock down, *then*
> > > free the objects it'd been using.  IOW,
> > 
> > I wondered about that but a lot of places do it the other way around.
> > So maybe the expected order should be documented somewhere.
> 
> ???
> 
> "If you are holding internal references to dentries/inodes/etc., drop them
> first; if you are going to free something that is used by filesystem
> methods, don't do that before the filesystem is shut down"
> 
> That's just common sense...  Which filesystems are doing that "the other
> way around"?

I think at least these below. Completely untested...

Signed-off-by: Christian Brauner (Microsoft) <brauner@...nel.org>
---
 arch/s390/hypfs/inode.c      |  3 +--
 fs/devpts/inode.c            |  2 +-
 fs/ramfs/inode.c             |  4 +++-
 security/selinux/selinuxfs.c | 12 ++++++------
 4 files changed, 11 insertions(+), 10 deletions(-)

diff --git a/arch/s390/hypfs/inode.c b/arch/s390/hypfs/inode.c
index 5c97f48cea91..d7d275ef132f 100644
--- a/arch/s390/hypfs/inode.c
+++ b/arch/s390/hypfs/inode.c
@@ -329,9 +329,8 @@ static void hypfs_kill_super(struct super_block *sb)
 		hypfs_delete_tree(sb->s_root);
 	if (sb_info && sb_info->update_file)
 		hypfs_remove(sb_info->update_file);
-	kfree(sb->s_fs_info);
-	sb->s_fs_info = NULL;
 	kill_litter_super(sb);
+	kfree(sb->s_fs_info);
 }
 
 static struct dentry *hypfs_create_file(struct dentry *parent, const char *name,
diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
index 4f25015aa534..78a9095e1748 100644
--- a/fs/devpts/inode.c
+++ b/fs/devpts/inode.c
@@ -509,10 +509,10 @@ static void devpts_kill_sb(struct super_block *sb)
 {
 	struct pts_fs_info *fsi = DEVPTS_SB(sb);
 
+	kill_litter_super(sb);
 	if (fsi)
 		ida_destroy(&fsi->allocated_ptys);
 	kfree(fsi);
-	kill_litter_super(sb);
 }
 
 static struct file_system_type devpts_fs_type = {
diff --git a/fs/ramfs/inode.c b/fs/ramfs/inode.c
index bc66d0173e33..bff49294e037 100644
--- a/fs/ramfs/inode.c
+++ b/fs/ramfs/inode.c
@@ -280,8 +280,10 @@ int ramfs_init_fs_context(struct fs_context *fc)
 
 static void ramfs_kill_sb(struct super_block *sb)
 {
-	kfree(sb->s_fs_info);
+	struct ramfs_fs_info *fsi = sb->s_fs_info;
+
 	kill_litter_super(sb);
+	kfree(fsi);
 }
 
 static struct file_system_type ramfs_fs_type = {
diff --git a/security/selinux/selinuxfs.c b/security/selinux/selinuxfs.c
index 8fcdd494af27..fb1dae422d93 100644
--- a/security/selinux/selinuxfs.c
+++ b/security/selinux/selinuxfs.c
@@ -96,9 +96,8 @@ static int selinux_fs_info_create(struct super_block *sb)
 	return 0;
 }
 
-static void selinux_fs_info_free(struct super_block *sb)
+static void selinux_fs_info_free(struct selinux_fs_info *fsi)
 {
-	struct selinux_fs_info *fsi = sb->s_fs_info;
 	int i;
 
 	if (fsi) {
@@ -107,8 +106,7 @@ static void selinux_fs_info_free(struct super_block *sb)
 		kfree(fsi->bool_pending_names);
 		kfree(fsi->bool_pending_values);
 	}
-	kfree(sb->s_fs_info);
-	sb->s_fs_info = NULL;
+	kfree(fsi);
 }
 
 #define SEL_INITCON_INO_OFFSET		0x01000000
@@ -2180,7 +2178,7 @@ static int sel_fill_super(struct super_block *sb, struct fs_context *fc)
 	pr_err("SELinux: %s:  failed while creating inodes\n",
 		__func__);
 
-	selinux_fs_info_free(sb);
+	selinux_fs_info_free(fsi);
 
 	return ret;
 }
@@ -2202,8 +2200,10 @@ static int sel_init_fs_context(struct fs_context *fc)
 
 static void sel_kill_sb(struct super_block *sb)
 {
-	selinux_fs_info_free(sb);
+	struct selinux_fs_info *fsi = sb->s_fs_info;
+
 	kill_litter_super(sb);
+	selinux_fs_info_free(fsi);
 }
 
 static struct file_system_type sel_fs_type = {
-- 
2.34.1