Re: [PATCH] fs, ipc: Use an asynchronous version of kern_unmount in IPC

[Salman Qazi <sqazi@google.com>]

On Wed, Feb 6, 2019 at 8:14 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> On Wed, Feb 06, 2019 at 11:53:54AM -0800, Salman Qazi wrote:
>
> > This patch solves the issue by removing synchronize_rcu from mq_put_mnt.
> > This is done by implementing an asynchronous version of kern_unmount.
> >
> > Since mntput() sleeps, it needs to be deferred to a work queue.
> >
> > Additionally, the callers of mq_put_mnt appear to be safe having
> > it behave asynchronously.  In particular, put_ipc_ns calls
> > mq_clear_sbinfo which renders the inode inaccessible for the purposes of
> > mqueue_create by making s_fs_info NULL.  This appears
> > to be the thing that prevents access while free_ipc_ns is taking place.
> > So, the unmount should be able to proceed lazily.
>
> Ugh...  I really doubt that it's correct.  The caller is
>                 mq_put_mnt(ns);
>                 free_ipc_ns(ns);
> and we have
> static void mqueue_evict_inode(struct inode *inode)
> {
>
> ...
>
>         ipc_ns = get_ns_from_inode(inode);
>
> with
>
> static struct ipc_namespace *get_ns_from_inode(struct inode *inode)
> {
>         struct ipc_namespace *ns;
>
>         spin_lock(&mq_lock);
>         ns = __get_ns_from_inode(inode);
>         spin_unlock(&mq_lock);
>         return ns;
> }
>
> and
>
> static inline struct ipc_namespace *__get_ns_from_inode(struct inode *inode)
> {
>         return get_ipc_ns(inode->i_sb->s_fs_info);
> }
>
> with ->s_fs_info being the ipc_namespace we are freeing after mq_put_ns()
>
> Are you saying that get_ipc_ns() after free_ipc_ns() is safe?  Because
> ->evict_inode() *IS* called on umount.  What happens to your patch if
> there was a regular file left on that filesystem?
>
> Smells like a memory corruptor...

Actually, the full context in the caller is

        if (refcount_dec_and_lock(&ns->count, &mq_lock)) {
                mq_clear_sbinfo(ns);
                spin_unlock(&mq_lock);
                mq_put_mnt(ns);
                free_ipc_ns(ns);
        }

And

void mq_clear_sbinfo(struct ipc_namespace *ns)
{
        ns->mq_mnt->mnt_sb->s_fs_info = NULL;
}

Therefore, s_fs_info should be NULL before we proceed to unmount.  So,
as far as I know, it should not be possible to find the ipc_namespace
from the mount.