Re: [PATCH v27 03/21] vfs: Add MAY_DELETE_SELF and MAY_DELETE_CHILD permission flags

["J. Bruce Fields" <bfields@fieldses.org>]

On Fri, Dec 02, 2016 at 10:57:42AM +0100, Miklos Szeredi wrote:
> On Tue, Oct 11, 2016 at 2:50 PM, Andreas Gruenbacher
> <agruenba@redhat.com> wrote:
> > Normally, deleting a file requires MAY_WRITE access to the parent
> > directory.  With richacls, a file may be deleted with MAY_DELETE_CHILD access
> > to the parent directory or with MAY_DELETE_SELF access to the file.
> >
> > To support that, pass the MAY_DELETE_CHILD mask flag to inode_permission()
> > when checking for delete access inside a directory, and MAY_DELETE_SELF
> > when checking for delete access to a file itself.
> >
> > The MAY_DELETE_SELF permission overrides the sticky directory check.
> 
> And MAY_DELETE_SELF seems totally inappropriate to any kind of rename,
> since from the point of view of the inode we are not doing anything at
> all.  The modifications are all in the parent(s), and that's where the
> permission checks need to be.

I'm having a hard time finding an authoritative reference here (Samba
people might be able to help), but my understanding is that Windows
gives this a meaning something like "may I delete a link to this file".

(And not even "may I delete the *last* link to this file", which might
also sound more logical.)

--b.

> 
> > @@ -2780,14 +2780,20 @@ static int may_delete_or_replace(struct inode *dir, struct dentry *victim,
> >         BUG_ON(victim->d_parent->d_inode != dir);
> >         audit_inode_child(dir, victim, AUDIT_TYPE_CHILD_DELETE);
> >
> > -       error = inode_permission(dir, mask);
> > +       error = inode_permission(dir, mask | MAY_WRITE | MAY_DELETE_CHILD);
> > +       if (!error && check_sticky(dir, inode))
> > +               error = -EPERM;
> > +       if (error && IS_RICHACL(inode) &&
> > +           inode_permission(inode, MAY_DELETE_SELF) == 0 &&
> > +           inode_permission(dir, mask) == 0)
> > +               error = 0;
> 
> Why is MAY_WRITE missing here?  Everything not aware of
> MAY_DELETE_SELF (e.g. LSMs) will still need MAY_WRITE otherwise this
> is going to be a loophole.
> 
> Thanks,
> Miklos
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