[PATCH v3 2/7] userns: Simpilify MNT_NODEV handling.

[Andy Lutomirski]

On Sep 16, 2015 6:01 PM, "Eric W. Biederman" <ebiederm at xmission.com> wrote:
>
> Andy Lutomirski <luto at amacapital.net> writes:
>
> > On Wed, Sep 16, 2015 at 1:02 PM, Seth Forshee
> > <seth.forshee at canonical.com> wrote:
> >> From: "Eric W. Biederman" <ebiederm at xmission.com>
> >>
> >> - Consolidate the testing if a device node may be opened in a new
> >>   function may_open_dev.
> >>
> >> - Move the check for allowing access to device nodes on filesystems
> >>   not mounted in the initial user namespace from mount time to open
> >>   time and include it in may_open_dev.
> >>
> >> This set of changes removes the implicit adding of MNT_NODEV which
> >> simplifies the logic in fs/namespace.c and removes a potentially
> >> problematic user visible difference in how normal and unprivileged
> >> mount namespaces work.
> >>
> >> Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>
> >
> >> -               /* Only in special cases allow devices from mounts
> >> -                * created outside the initial user namespace.
> >> -                */
> >> -               if (!(type->fs_flags & FS_USERNS_DEV_MOUNT)) {
> >> -                       flags |= MS_NODEV;
> >> -                       mnt_flags |= MNT_NODEV | MNT_LOCK_NODEV;
> >> -               }
> >
> > This is an ABI change.  It's probably okay, but I think the commit
> > message should make it clear what's happening.
>
> You mean it should include in big flashing neon letters
>             ***REGRESSION FIX***
> ?
>
> It is longer in coming than I had hoped.  But that is part of the reason
> I did not fix the security hole this way.  Getting the s_user_ns stuff
> just so has been non-trivial.
>
> I do agree that because this is a user visible change we do need to keep
> our eyes peeled for pieces of userspace software that may depend on the
> exact details of the current behavior.

Yeah.  "Removes a potentially problematic user visible difference"
sounds like the difference has been there forever.   If it needs to
get backported, people will appreciate it.

--Andy