Re: [PATCH 0/2] capability conversion fixes

From: Miklos Szeredi
Date: Wed Jan 20 2021 - 02:42:08 EST

On Tue, Jan 19, 2021 at 10:15 PM Eric W. Biederman
<ebiederm@xxxxxxxxxxxx> wrote:
> Miklos Szeredi <mszeredi@xxxxxxxxxx> writes:
> > It turns out overlayfs is actually okay wrt. mutliple conversions, because
> > it uses the right context for lower operations. I.e. before calling
> > vfs_{set,get}xattr() on underlying fs, it overrides creds with that of the
> > mounter, so the current user ns will now match that of
> > overlay_sb->s_user_ns, meaning that the caps will be converted to just the
> > right format for the next layer
> >
> > OTOH ecryptfs, which is the only other one affected by commit 7c03e2cda4a5
> > ("vfs: move cap_convert_nscap() call into vfs_setxattr()") needs to be
> > fixed up, since it doesn't do the cap override thing that overlayfs does.
> >
> > I don't have an ecryptfs setup, so untested, but it's a fairly trivial
> > change.
> >
> > My other observation was that cap_inode_getsecurity() messes up conversion
> > of caps in more than one case. This is independent of the overlayfs user
> > ns enablement but affects it as well.
> >
> > Maybe we can revisit the infrastructure improvements we discussed, but I
> > think these fixes are more appropriate for the current cycle.
> I mostly agree. Fixing the bugs in a back-portable way is important.
> However we need to sort out the infrastructure, and implementation.
> As far as I can tell it is only the fact that overlayfs does not support
> the new mount api aka fs_context that allows this fix to work and be
> correct.
> I believe the new mount api would allow specifying a different userns
> thatn curent_user_ns for the overlay filesystem and that would break
> this.

This is a valid concern. I'll add a WARN_ON() to make sure that
whenever this changes it doesn't go unnoticed.

Fixing it would also be easy: just update creds->user_ns field to
that of sb->s_user_ns in ovl_fill_super(). For now I'll go with the
WARNING though, since this cannot be tested.