Re: [PATCH] selinux: always allow mounting submounts

[Ondrej Mosnacek <omosnace@redhat.com>]

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 was hoping to get some independent testing after porting the test to
the selinux-testsuite... (But I get it, the burden of proof is on my
side...)

>
> > Fixes: 93faccbbfa95 ("fs: Better permission checking for submounts")
> > Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
> > ---
> >  security/selinux/hooks.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> > index 7ce683259357..7ce012d9ec51 100644
> > --- a/security/selinux/hooks.c
> > +++ b/security/selinux/hooks.c
> > @@ -2934,7 +2934,7 @@ static int selinux_sb_kern_mount(struct super_block *sb, int flags, void *data)
> >                 return rc;
> >
> >         /* Allow all mounts performed by the kernel */
> > -       if (flags & MS_KERNMOUNT)
> > +       if (flags & (MS_KERNMOUNT | MS_SUBMOUNT))
> >                 return 0;
> >
> >         ad.type = LSM_AUDIT_DATA_DENTRY;
>
> --
> paul moore
> www.paul-moore.com
--
Ondrej Mosnacek <omosnace at redhat dot com>
Associate Software Engineer, Security Technologies
Red Hat, Inc.