Re: [PATCH linux-next] mqueue: fix IPC namespace use-after-free

[Al Viro]

On Tue, Dec 19, 2017 at 11:14:40AM +0100, Giuseppe Scrivano wrote:
> mqueue_evict_inode() doesn't access the ipc namespace if it was
> already freed.  It can happen if in a new IPC namespace the inode was
> created without a prior mq_open() which creates the vfsmount used to
> access the superblock from mq_clear_sbinfo().
> 
> Keep a direct pointer to the superblock used by the inodes so we can
> correctly reset the reference to the IPC namespace being destroyed.
> 
> Bug introduced with 9c583773d03633 ("ipc, mqueue: lazy call
> kern_mount_data in new namespaces")

And just what will happen in the same scenario if you mount the damn
thing in userland without ever calling mq_open(), touch a file there,
then unmount and then leave the ipc namespace?