Linux Virtualization - May 2021 - [PATCH v4 1/5] fuse: Fix leak in fuse_dentry_automount() error path

On Fri, 21 May 2021 09:54:19 +0200
Miklos Szeredi <miklos at szeredi.hu> wrote:
> On Thu, 20 May 2021 at 21:45, Al Viro <viro at zeniv.linux.org.uk>
wrote:
> >
> > On Thu, May 20, 2021 at 05:46:50PM +0200, Greg Kurz wrote:
> > > Some rollback was forgotten during the addition of crossmounts.
> >
> > Have you actually tested that?  Because I strongly suspect that
> > by that point the ownership of fc and fm is with sb and those
> > should be taken care of by deactivate_locked_super().
> 
> Not quite.  Patch looks correct because destruction of fm is done in
> fuse_put_super(), which only gets called if the sb initialization gets
> as far as setting up sb->s_root, which only happens after the
> successful fuse_fill_super_submount() call in this case.
> 
> Doing the destruction from the various ->kill_sb() instances instead
> of from ->put_super() would also fix this, but I'm not quite sure
that
> that would be any cleaner.
> 
As saying in the answer I've just posted, a failure in
fuse_fill_super_submount() causes an actual crash because
fuse_mount_remove() logically assumes fm to already be in
fc->mounts, which isn't the case at this point.

In the root mount case, this is handled by taking back
the ownership on fm, i.e. do the rollback *and* clear
sb->s_fs_info. It seems that the same should be done
for submounts.
> Thanks,
> Miklos