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Date:   Wed, 17 Aug 2022 16:21:11 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Christian Brauner <brauner@kernel.org>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.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] binderfs: rework superblock destruction
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On Wed, Aug 17, 2022 at 04:51:44PM +0200, Christian Brauner wrote:

> 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);

UAF, that - *sb gets freed by the time you try to fetch sb->s_fs_info...
Fetch the pointer first, then destroy the object you've fetched it
from, then free what it points to...

> 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);
>  }
>  

That one's fine.

>  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);
>  }

Cosmetical, really - see another posting in the same thread.

>  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);
>  }

A real bug, but an incomplete fix - you've just gotten yourself a double-free;
failure in sel_fill_super() has no need to do selinux_fs_info_free() now.