Re: [PATCH] nilfs2: fix use-after-free bug in nilfs_mdt_destroy()

[Al Viro]

On Tue, Aug 16, 2022 at 02:51:14AM +0900, Ryusuke Konishi wrote:
> In alloc_inode(), inode_init_always() could return -ENOMEM if
> security_inode_alloc() fails.  If this happens for nilfs2,
> nilfs_free_inode() is called without initializing inode->i_private and
> nilfs_free_inode() wrongly calls nilfs_mdt_destroy(), which frees
> uninitialized inode->i_private and can trigger a crash.
> 
> Fix this bug by initializing inode->i_private in nilfs_alloc_inode().
> 
> Link: https://lkml.kernel.org/r/CAFcO6XOcf1Jj2SeGt=jJV59wmhESeSKpfR0omdFRq+J9nD1vfQ@xxxxxxxxxxxxxx
> Link: https://lkml.kernel.org/r/20211011030956.2459172-1-mudongliangabcd@xxxxxxxxx
> Reported-by: butt3rflyh4ck <butterflyhuangxx@xxxxxxxxx>
> Reported-by: Hao Sun <sunhao.th@xxxxxxxxx>
> Reported-by: Jiacheng Xu <stitch@xxxxxxxxxx>
> Reported-by: Mudong Liang <mudongliangabcd@xxxxxxxxx>
> Signed-off-by: Ryusuke Konishi <konishi.ryusuke@xxxxxxxxx>
> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> ---
>  fs/nilfs2/super.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/fs/nilfs2/super.c b/fs/nilfs2/super.c
> index ba108f915391..aca5614f1b44 100644
> --- a/fs/nilfs2/super.c
> +++ b/fs/nilfs2/super.c
> @@ -159,6 +159,7 @@ struct inode *nilfs_alloc_inode(struct super_block *sb)
>  	ii->i_cno = 0;
>  	ii->i_assoc_inode = NULL;
>  	ii->i_bmap = &ii->i_bmap_data;
> +	ii->vfs_inode.i_private = NULL;
>  	return &ii->vfs_inode;
>  }

FWIW, I think it's better to deal with that in inode_init_always(), but
not just moving ->i_private initialization up - we ought to move
security_inode_alloc() to the very end.  No sense playing whack-a-mole
with further possible bugs of that sort...