Re: [PATCH] selinux: always allow mounting submounts

[Paul Moore <paul@paul-moore.com>]

On Wed, Nov 21, 2018 at 10:38 AM Ondrej Mosnacek <omosnace@redhat.com> wrote:
> On Wed, Nov 21, 2018 at 1:41 PM Ondrej Mosnacek <omosnace@redhat.com> wrote:
> > On Tue, Nov 20, 2018 at 11:09 PM Paul Moore <paul@paul-moore.com> wrote:
> > > On Fri, Nov 16, 2018 at 8:12 AM Ondrej Mosnacek <omosnace@redhat.com> wrote:
> > > > If a superblock has the MS_SUBMOUNT flag set, we should always allow
> > > > mounting it. These mounts are done automatically by the kernel either as
> > > > part of mounting some parent mount (e.g. debugfs always mounts tracefs
> > > > under "tracing" for compatibility) or they are mounted automatically as
> > > > needed on subdirectory accesses (e.g. NFS crossmnt mounts). Since such
> > > > automounts are either an implicit consequence of the parent mount (which
> > > > is already checked) or they can happen during regular accesses (where it
> > > > doesn't make sense to check against the current task's context), the
> > > > mount permission check should be skipped for them.
> > > >
> > > > Without this patch, attempts to access contents of an automounted
> > > > directory can cause unexpected SELinux denials.
> > > >
> > > > In the current kernel tree, the MS_SUBMOUNT flag is set only via
> > > > vfs_submount(), which is called only from the following places:
> > > >  - AFS, when automounting special "symlinks" referencing other cells
> > > >  - CIFS, when automounting "referrals"
> > > >  - NFS, when automounting subtrees
> > > >  - debugfs, when automounting tracefs
> > > >
> > > > In all cases the submounts are meant to be transparent to the user and
> > > > it makes sense that if mounting the master is allowed, then so should be
> > > > the automounts. Note that CAP_SYS_ADMIN capability checking is already
> > > > skipped for (SB_KERNMOUNT|SB_SUBMOUNT) in:
> > > >  - sget_userns() in fs/super.c:
> > > >         if (!(flags & (SB_KERNMOUNT|SB_SUBMOUNT)) &&
> > > >             !(type->fs_flags & FS_USERNS_MOUNT) &&
> > > >             !capable(CAP_SYS_ADMIN))
> > > >                 return ERR_PTR(-EPERM);
> > > >  - sget() in fs/super.c:
> > > >         /* Ensure the requestor has permissions over the target filesystem */
> > > >         if (!(flags & (SB_KERNMOUNT|SB_SUBMOUNT)) && !ns_capable(user_ns, CAP_SYS_ADMIN))
> > > >                 return ERR_PTR(-EPERM);
> > > >
> > > > Verified internally on patched RHEL 7.6 with a reproducer using
> > > > NFS+httpd and selinux-tesuite.
> > >
> > > I think this all sounds reasonable, but please verify this with an
> > > upstream kernel.  Upstream our focus is on the upstream kernel
> > > (surprise!), downstream RHEL is your responsibility, not ours :)
> >
> > I tested on RHEL because that's what I can do most conveniently. I
> > don't have a very good workflow/environment for complex testing on
> > upstream right now. I don't expect the results to be any different on
> > the upstream kernel, but I understand your concern. I have been
> > thinking about some patch testing automation using Fedora Rawhide (I
> > hope that's close enough to upstream at least :), so I guess it's time
> > to get scriptin'...
>
> I have now tested it on Fedora Rawhide with a scratch kernel with this
> patch applied [1] (x86_64 only). I ran the whole selinux-testsuite
> with the submount test [2] and everything passed (except for the known
> overlay failures and skipped binder test) ...

Merged into selinux/next, thanks.

-- 
paul moore
www.paul-moore.com