On Wed, 2017-07-12 at 15:38 +0200, Dominick Grift wrote:
> On Wed, Jul 12, 2017 at 03:30:25PM +0200, Dominick Grift wrote:
> > On Wed, Jul 12, 2017 at 09:01:48AM -0400, Stephen Smalley wrote:
> > > On Tue, 2017-07-11 at 22:44 +0200, Dominick Grift wrote:
> > > > On Tue, Jul 11, 2017 at 04:23:29PM -0400, Stephen Smalley
> > > > wrote:
> > > > > On Tue, 2017-07-11 at 22:10 +0200, Dominick Grift wrote:
> > > > > > On Tue, Jul 11, 2017 at 10:05:36PM +0200, Dominick Grift
> > > > > > wrote:
> > > > > > > On Tue, Jul 11, 2017 at 03:52:52PM -0400, Stephen Smalley
> > > > > > > wrote:
> > > > > > > > On Mon, 2017-07-10 at 16:25 -0400, Stephen Smalley
> > > > > > > > wrote:
> > > > > > > > > As systemd ramps up enabling NoNewPrivileges (either
> > > > > > > > > explicitly
> > > > > > > > > in
> > > > > > > > > service unit files or as a side effect of other
> > > > > > > > > security-
> > > > > > > > > related
> > > > > > > > > settings in service unit files), we're increasingly
> > > > > > > > > running
> > > > > > > > > afoul of
> > > > > > > > > its interactions with SELinux.
> > > > > > > > >
> > > > > > > > > The end result is bad for the security of both
> > > > > > > > > SELinux-
> > > > > > > > > disabled
> > > > > > > > > and
> > > > > > > > > SELinux-enabled systems. Packagers have to turn off
> > > > > > > > > these
> > > > > > > > > options in the unit files to preserve SELinux domain
> > > > > > > > > transitions. For
> > > > > > > > > users who choose to disable SELinux, this means that
> > > > > > > > > they
> > > > > > > > > miss
> > > > > > > > > out on
> > > > > > > > > at least having the systemd-supported
> > > > > > > > > protections. For
> > > > > > > > > users
> > > > > > > > > who
> > > > > > > > > keep
> > > > > > > > > SELinux enabled, they may still be missing out on
> > > > > > > > > some
> > > > > > > > > protections
> > > > > > > > > because it isn't necessarily guaranteed that the
> > > > > > > > > SELinux
> > > > > > > > > policy
> > > > > > > > > for
> > > > > > > > > that service provides the same protections in all
> > > > > > > > > cases.
> > > > > > > > >
> > > > > > > > > Our options seem to be:
> > > > > > > > >
> > > > > > > > > 1) Just keep on the way we are now, i.e. packagers
> > > > > > > > > have to
> > > > > > > > > remove
> > > > > > > > > default protection settings from upstream package
> > > > > > > > > unit
> > > > > > > > > files in
> > > > > > > > > order
> > > > > > > > > to have them work with SELinux (and not just
> > > > > > > > > NoNewPrivileges=
> > > > > > > > > itself; increasingly systemd is enabling NNP as a
> > > > > > > > > side
> > > > > > > > > effect
> > > > > > > > > of
> > > > > > > > > other
> > > > > > > > > unit file options, even seemingly unrelated ones like
> > > > > > > > > PrivateDevices).
> > > > > > > > > SELinux-disabled users lose entirely, SELinux-enabled
> > > > > > > > > users
> > > > > > > > > may
> > > > > > > > > lose
> > > > > > > > > (depending on whether SELinux policy provides
> > > > > > > > > equivalent or
> > > > > > > > > better guarantees).
> > > > > > > > >
> > > > > > > > > 2) Change systemd to automatically disable NNP on
> > > > > > > > > SELinux-
> > > > > > > > > enabled
> > > > > > > > > systems. Unit files can be left unmodified from
> > > > > > > > > upstream. SELinux-
> > > > > > > > > disabled users win. SELinux-enabled users may lose.
> > > > > > > > >
> > > > > > > > > 3) Try to use typebounds, since we allow bounded
> > > > > > > > > transitions
> > > > > > > > > under
> > > > > > > > > NNP.
> > > > > > > > > Unit files can be left unmodified from upstream.
> > > > > > > > > SELinux-
> > > > > > > > > disabled
> > > > > > > > > users
> > > > > > > > > win. SELinux-enabled users get to benefit from
> > > > > > > > > systemd-
> > > > > > > > > provided
> > > > > > > > > protections. However, this option is impractical to
> > > > > > > > > implement
> > > > > > > > > in
> > > > > > > > > policy
> > > > > > > > > currently, since typebounds requires us to ensure
> > > > > > > > > that each
> > > > > > > > > domain is
> > > > > > > > > allowed everything all of its descendant domains are
> > > > > > > > > allowed,
> > > > > > > > > and
> > > > > > > > > this
> > > > > > > > > has to be repeated for the entire chain of domain
> > > > > > > > > transitions. There
> > > > > > > > > is
> > > > > > > > > no way to clone all allow rules from children to the
> > > > > > > > > parent
> > > > > > > > > in
> > > > > > > > > policy
> > > > > > > > > currently, and it is doubtful that doing so would be
> > > > > > > > > desirable
> > > > > > > > > even
> > > > > > > > > if
> > > > > > > > > it were practical, as it requires leaking permissions
> > > > > > > > > to
> > > > > > > > > objects and
> > > > > > > > > operations into parent domains that could weaken
> > > > > > > > > their own
> > > > > > > > > security
> > > > > > > > > in
> > > > > > > > > order to allow them to the children (e.g. if a child
> > > > > > > > > requires
> > > > > > > > > execmem
> > > > > > > > > permission, then so does the parent; if a child
> > > > > > > > > requires
> > > > > > > > > read
> > > > > > > > > to a
> > > > > > > > > symbolic
> > > > > > > > > link or temporary file that it can write, then so
> > > > > > > > > does the
> > > > > > > > > parent,
> > > > > > > > > ...).
> > > > > > > > >
> > > > > > > > > 4) Decouple NNP from SELinux transitions, so that we
> > > > > > > > > don't
> > > > > > > > > have
> > > > > > > > > to
> > > > > > > > > make a choice between them. Introduce a new policy
> > > > > > > > > capability
> > > > > > > > > that
> > > > > > > > > causes the ability to transition under NNP to be
> > > > > > > > > based on a
> > > > > > > > > new
> > > > > > > > > permission
> > > > > > > > > check between the old and new contexts rather than
> > > > > > > > > typebounds. Domain
> > > > > > > > > transitions can then be allowed in policy without
> > > > > > > > > requiring
> > > > > > > > > the
> > > > > > > > > parent
> > > > > > > > > to be a strict superset of all of its children. The
> > > > > > > > > rationale
> > > > > > > > > for
> > > > > > > > > this
> > > > > > > > > divergence from NNP behavior for capabilities is that
> > > > > > > > > SELinux
> > > > > > > > > permissions
> > > > > > > > > are substantially broader than just capabilities
> > > > > > > > > (they
> > > > > > > > > express
> > > > > > > > > a full
> > > > > > > > > access matrix, not just privileges) and can only be
> > > > > > > > > used to
> > > > > > > > > further
> > > > > > > > > restrict capabilities, not grant them beyond what is
> > > > > > > > > already
> > > > > > > > > permitted.
> > > > > > > > > Unit files can be left unmodified from
> > > > > > > > > upstream. SELinux-
> > > > > > > > > disabled
> > > > > > > > > users
> > > > > > > > > win. SELinux-enabled users can benefit from systemd-
> > > > > > > > > provided
> > > > > > > > > protections
> > > > > > > > > and policy won't need to radically change.
> > > > > > > > >
> > > > > > > > > This change takes the last approach above, as it
> > > > > > > > > seems to
> > > > > > > > > be
> > > > > > > > > the
> > > > > > > > > best option.
> > > > > > > > >
> > > > > > > > > Signed-off-by: Stephen Smalley <s...@tycho.nsa.gov>
> > > > > > > > > ---
> > > > > > > > > security/selinux/hooks.c | 41
> > > > > > > > > ++++++++++++++++++++++++---
> > > > > > > > > ----------
> > > > > > > > > security/selinux/include/classmap.h | 2 +-
> > > > > > > > > security/selinux/include/security.h | 2 ++
> > > > > > > > > security/selinux/ss/services.c | 7 ++++++-
> > > > > > > > > 4 files changed, 36 insertions(+), 16 deletions(-)
> > > > > > > > >
> > > > > > > > > diff --git a/security/selinux/hooks.c
> > > > > > > > > b/security/selinux/hooks.c
> > > > > > > > > index 3a06afb..f0c11c2 100644
> > > > > > > > > --- a/security/selinux/hooks.c
> > > > > > > > > +++ b/security/selinux/hooks.c
> > > > > > > > > @@ -2326,24 +2326,37 @@ static int
> > > > > > > > > check_nnp_nosuid(const
> > > > > > > > > struct
> > > > > > > > > linux_binprm *bprm,
> > > > > > > > > return 0; /* No change in
> > > > > > > > > credentials */
> > > > > > > > >
> > > > > > > > > /*
> > > > > > > > > - * The only transitions we permit under NNP
> > > > > > > > > or
> > > > > > > > > nosuid
> > > > > > > > > - * are transitions to bounded SIDs, i.e.
> > > > > > > > > SIDs that
> > > > > > > > > are
> > > > > > > > > - * guaranteed to only be allowed a subset of
> > > > > > > > > the
> > > > > > > > > permissions
> > > > > > > > > - * of the current SID.
> > > > > > > > > + * If the policy enables the nnp_transition
> > > > > > > > > policy
> > > > > > > > > capability,
> > > > > > > > > + * then we permit transitions under NNP or
> > > > > > > > > nosuid
> > > > > > > > > if
> > > > > > > > > the
> > > > > > > > > + * policy explicitly allows nnp_transition
> > > > > > > > > permission
> > > > > > > > > between
> > > > > > > > > + * the old and new contexts.
> > > > > > > > > */
> > > > > > > > > - rc = security_bounded_transition(old_tsec-
> > > > > > > > > >sid,
> > > > > > > > > new_tsec-
> > > > > > > > > > sid);
> > > > > > > > >
> > > > > > > > > - if (rc) {
> > > > > > > > > + if (selinux_policycap_nnptransition) {
> > > > > > > > > + rc = avc_has_perm(old_tsec->sid,
> > > > > > > > > new_tsec-
> > > > > > > > > > sid,
> > > > > > > > >
> > > > > > > > > + SECCLASS_PROCESS,
> > > > > > > > > + PROCESS__NNP_TRANS
> > > > > > > > > ITION,
> > > > > > > > > NULL);
> > > > > > > > > + if (!rc)
> > > > > > > > > + return 0;
> > > > > > > > > + } else {
> > > > > > > > > /*
> > > > > > > > > - * On failure, preserve the errno
> > > > > > > > > values
> > > > > > > > > for
> > > > > > > > > NNP vs
> > > > > > > > > nosuid.
> > > > > > > > > - * NNP: Operation not permitted for
> > > > > > > > > caller.
> > > > > > > > > - * nosuid: Permission denied to
> > > > > > > > > file.
> > > > > > > > > + * Otherwise, the only transitions
> > > > > > > > > we
> > > > > > > > > permit
> > > > > > > > > under
> > > > > > > > > NNP or nosuid
> > > > > > > > > + * are transitions to bounded SIDs,
> > > > > > > > > i.e.
> > > > > > > > > SIDs
> > > > > > > > > that
> > > > > > > > > are
> > > > > > > > > + * guaranteed to only be allowed a
> > > > > > > > > subset
> > > > > > > > > of
> > > > > > > > > the
> > > > > > > > > permissions
> > > > > > > > > + * of the current SID.
> > > > > > > > > */
> > > > > > > > > - if (nnp)
> > > > > > > > > - return -EPERM;
> > > > > > > > > - else
> > > > > > > > > - return -EACCES;
> > > > > > > > > + rc =
> > > > > > > > > security_bounded_transition(old_tsec-
> > > > > > > > > > sid,
> > > > > > > > >
> > > > > > > > > new_tsec->sid);
> > > > > > > > > + if (!rc)
> > > > > > > > > + return 0;
> > > > > > > >
> > > > > > > > NB: As currently written, this logic means that if you
> > > > > > > > enable
> > > > > > > > the
> > > > > > > > new
> > > > > > > > policy capability, the only way to allow a transition
> > > > > > > > under
> > > > > > > > NNP
> > > > > > > > is by
> > > > > > > > allowing nnp_transition permission between the old and
> > > > > > > > new
> > > > > > > > domains. The
> > > > > > > > alternative would be to fall back to checking for a
> > > > > > > > bounded
> > > > > > > > SID
> > > > > > > > if
> > > > > > > > nnp_transition permission is not allowed. The former
> > > > > > > > approach
> > > > > > > > has
> > > > > > > > the
> > > > > > > > advantage of being simpler (only a single check with a
> > > > > > > > single
> > > > > > > > audit
> > > > > > > > message), but means that you can't mix usage of bounded
> > > > > > > > SIDs
> > > > > > > > and
> > > > > > > > nnp_transition permission as a means of allowing a
> > > > > > > > transitions
> > > > > > > > under
> > > > > > > > NNP within the same policy. The latter approach
> > > > > > > > provides
> > > > > > > > more
> > > > > > > > flexibility / compatibility but will end up producing
> > > > > > > > two
> > > > > > > > audit
> > > > > > > > messages in the case where the transition is denied by
> > > > > > > > both
> > > > > > > > checks: an
> > > > > > > > AVC message for the permission denial and a SELINUX_ERR
> > > > > > > > message
> > > > > > > > for the
> > > > > > > > security_bounded_transition failure, which might be
> > > > > > > > confusing
> > > > > > > > to
> > > > > > > > users
> > > > > > > > (but probably not; they'll likely just feed the AVC
> > > > > > > > through
> > > > > > > > audit2allow
> > > > > > > > and be done with it). Thoughts?
> > > > > > >
> > > > > > > I think the current implementation is fine if i
> > > > > > > understand
> > > > > > > correctly:
> > > > > > >
> > > > > > > 1. With the policy capability disabled the behavior
> > > > > > > doesnt
> > > > > > > change,
> > > > > > > so if you dont want the current behavior (type_bounds)
> > > > > > > then
> > > > > > > just to
> > > > > > > enable the polcap
> > > > > > > 2. If you enable the policy capability then you decided
> > > > > > > to
> > > > > > > leverage
> > > > > > > nnp_transition instead of the current behavior
> > > > > > >
> > > > > > > Theres plenty flexibility with this approach i would
> > > > > > > argue
> > > > > >
> > > > > > Hmm. that came out wrong:
> > > > > >
> > > > > > 1. without nnptransition polcap: everything stays the same
> > > > > > 2. with nnptransition polcap: you choose nnp_transition
> > > > > > over
> > > > > > current
> > > > > > type_bounds behavior
> > > > >
> > > > > Yes, that's correct. It is somewhat limiting in that if you
> > > > > want to
> > > > > leverage nnp_transition at all, you have to give up using
> > > > > typebounds
> > > > > entirely for allowing NNP transitions. So, let's say there
> > > > > is an
> > > > > existing policy module that defines a typebounds in order to
> > > > > allow
> > > > > a
> > > > > NNP transition, and then another policy module decides to
> > > > > enable
> > > > > the
> > > > > policy capability and leverage nnp_transition permission to
> > > > > allow
> > > > > another NNP transition that wouldn't work under
> > > > > typebounds. The
> > > > > first
> > > > > one will break under the former approach, while it would keep
> > > > > working
> > > > > under the latter approach. Not sure if that's a real concern
> > > > > or
> > > > > just a
> > > > > contrived one. Let's say someone writes a third party policy
> > > > > module
> > > > > using typebounds for this purpose on Fedora today, and then
> > > > > updates
> > > > > to
> > > > > a new version of Fedora that enables the policy capability
> > > > > and
> > > > > leverages nnp_transition in its policy to allow such
> > > > > transitions. Now
> > > > > that third party policy module will stop working (it would
> > > > > need to
> > > > > be
> > > > > changed to allow nnp_transition explicitly).
> > > >
> > > > Would this affect the requirement of typebounds by for example
> > > > mod_selinux? I think that apache module also requires
> > > > typebounds but
> > > > not for NNP AFAIK (that stuff predates it i believe)
> > >
> > > No, this doesn't affect that. That's a
> > > security_bounded_transition()
> > > check in selinux_setprocattr() for writes to
> > > /proc/self/attr/current if
> > > the process is multi-threaded.
> > >
> > > > I prefer this implementation if this implementation is
> > > > reasonably
> > > > possible. I dont believe that there are any third party modules
> > > > that
> > > > support NNP (its too un-usable and hard to write)
> > > >
> > > > I wont be leveraging nnp_transition or type_bounds for NNP BTW.
> > > > I
> > > > will just contrinue removing the NNP= for systemd service
> > > > units.
> > >
> > > Note that it isn't sufficient to just remove NoNewPrivileges=
> > > from the
> > > unit file; you also have to remove any other options that
> > > implicitly
> > > enable NNP, which seems to be growing. For example, any of the
> > > following will now enable NNP too as a side effect:
> > > SystemCallFilter=, SystemCallArchitectures=,
> > > RestrictAddressFamilies=,
> > > RestrictNamespaces=, PrivateDevices=, ProtectKernelTunables=,
> > > ProtectKernelModules=, MemoryDenyWriteExecute=, or
> > > RestrictRealtime=
> >
> > Are you sure because if that was the case I probably would have
> > noticed?
>
> Hmm.. man systemd.exec does say that its implied. It is strange that
> i haven't encountered it.
>
> Why would nnp have to be implied for for example
> ProtectKernelTunables?
>
> This makes things really ugly ...
That's why I'm trying to resolve it on the SELinux side of things,
because I don't think it is workable to just disable all of these
options in unit files. And I don't have any control over systemd.
Looks like systemd only enables NNP if one of those options is enabled
_and_ the process lacks CAP_SYS_ADMIN. So you'll encounter it with
unit files that are configured to run as non-root or without
CAP_SYS_ADMIN.
I ran into this when someone raised a problem with radicale not
transitioning on the fedora selinux list and it turned out to be due to
PrivateDevices=true in combination with User=radicale in the unit file.
