[PATCH] LSM: add SafeSetID module that gates setid calls

[Micah Morton]

On Thu, Nov 1, 2018 at 10:08 AM Casey Schaufler <casey at schaufler-ca.com> wrote:
>
> On 11/1/2018 9:11 AM, Micah Morton wrote:
> > On Wed, Oct 31, 2018 at 11:07 PM Serge E. Hallyn <serge at hallyn.com> wrote:
> >> On Wed, Oct 31, 2018 at 09:02:45PM +0000, Serge E. Hallyn wrote:
> >>> Quoting mortonm at chromium.org (mortonm at chromium.org):
> >>>> From: Micah Morton <mortonm at chromium.org>
> >>>>
> >>>> SafeSetID gates the setid family of syscalls to restrict UID/GID
> >>>> transitions from a given UID/GID to only those approved by a
> >>>> system-wide whitelist. These restrictions also prohibit the given
> >>>> UIDs/GIDs from obtaining auxiliary privileges associated with
> >>>> CAP_SET{U/G}ID, such as allowing a user to set up user namespace UID
> >>>> mappings. For now, only gating the set*uid family of syscalls is
> >>>> supported, with support for set*gid coming in a future patch set.
> >>>>
> >>>> Signed-off-by: Micah Morton <mortonm at chromium.org>
> >>>> ---
> >>>>
> >>>> NOTE: See the TODO above setuid_syscall() in lsm.c for an aspect of this
> >>>> code that likely needs improvement before being an acceptable approach.
> >>>> I'm specifically interested to see if there are better ideas for how
> >>>> this could be done.
> >>>>
> >>>>  Documentation/admin-guide/LSM/SafeSetID.rst |  94 ++++++
> >>>>  Documentation/admin-guide/LSM/index.rst     |   1 +
> >>>>  arch/Kconfig                                |   5 +
> >>>>  arch/arm/Kconfig                            |   1 +
> >>>>  arch/arm64/Kconfig                          |   1 +
> >>>>  arch/x86/Kconfig                            |   1 +
> >>>>  security/Kconfig                            |   1 +
> >>>>  security/Makefile                           |   2 +
> >>>>  security/safesetid/Kconfig                  |  13 +
> >>>>  security/safesetid/Makefile                 |   7 +
> >>>>  security/safesetid/lsm.c                    | 334 ++++++++++++++++++++
> >>>>  security/safesetid/lsm.h                    |  30 ++
> >>>>  security/safesetid/securityfs.c             | 189 +++++++++++
> >>>>  13 files changed, 679 insertions(+)
> >>>>  create mode 100644 Documentation/admin-guide/LSM/SafeSetID.rst
> >>>>  create mode 100644 security/safesetid/Kconfig
> >>>>  create mode 100644 security/safesetid/Makefile
> >>>>  create mode 100644 security/safesetid/lsm.c
> >>>>  create mode 100644 security/safesetid/lsm.h
> >>>>  create mode 100644 security/safesetid/securityfs.c
> >>>>
> >>>> diff --git a/Documentation/admin-guide/LSM/SafeSetID.rst b/Documentation/admin-guide/LSM/SafeSetID.rst
> >>>> new file mode 100644
> >>>> index 000000000000..e7d072124424
> >>>> --- /dev/null
> >>>> +++ b/Documentation/admin-guide/LSM/SafeSetID.rst
> >>>> @@ -0,0 +1,94 @@
> >>>> +=========
> >>>> +SafeSetID
> >>>> +=========
> >>>> +SafeSetID is an LSM module that gates the setid family of syscalls to restrict
> >>>> +UID/GID transitions from a given UID/GID to only those approved by a
> >>>> +system-wide whitelist. These restrictions also prohibit the given UIDs/GIDs
> >>>> +from obtaining auxiliary privileges associated with CAP_SET{U/G}ID, such as
> >>>> +allowing a user to set up user namespace UID mappings.
> >>>> +
> >>>> +
> >>>> +Background
> >>>> +==========
> >>>> +In absence of file capabilities, processes spawned on a Linux system that need
> >>>> +to switch to a different user must be spawned with CAP_SETUID privileges.
> >>>> +CAP_SETUID is granted to programs running as root or those running as a non-root
> >>>> +user that have been explicitly given the CAP_SETUID runtime capability. It is
> >>>> +often preferable to use Linux runtime capabilities rather than file
> >>>> +capabilities, since using file capabilities to run a program with elevated
> >>>> +privileges opens up possible security holes since any user with access to the
> >>>> +file can exec() that program to gain the elevated privileges.
> >>> Not true, see inheritable capabilities.  You also might look at ambient
> >>> capabilities.
> >> So for example with pam_cap.so you could have your N uids each be given
> >> the desired pI, and assign the corrsponding fIs to the files they should
> >> be able to exec with privilege.  No other uids will run those files with
> >> privilege.  *1
> > Sorry, what are "pl" and "fls" here? "Privilege level" and "files"?
> >
> >> Can you give some more details about exactly how you see SafeSetID being
> >> used?
> > Sure. The main use case for this LSM is to allow a non-root program to
> > transition to other untrusted uids without full blown CAP_SETUID
> > capabilities. The non-root program would still need CAP_SETUID to do
> > any kind of transition, but the additional restrictions imposed by
> > this LSM would mean it is a "safer" version of CAP_SETUID since the
> > non-root program cannot take advantage of CAP_SETUID to do any
> > unapproved actions (i.e. setuid to uid 0 or create/enter new user
> > namespace). The higher level goal is to allow for uid-based sandboxing
> > of system services without having to give out CAP_SETUID all over the
> > place just so that non-root programs can drop to
> > even-further-non-privileged uids. This is especially relevant when one
> > non-root daemon on the system should be allowed to spawn other
> > processes as different uids, but its undesirable to give the daemon a
> > basically-root-equivalent CAP_SETUID.
>
> I don't want to sound stupid(er than usual), but it sounds like
> you could do all this using setuid bits prudently. Based on this
> description, I don't see that anything new is needed.

There are situations where setuid bits don't get the job done, as
there are many situations where a program just wants to call setuid as
part of its execution (or fork + setuid without exec), instead of
fork/exec'ing a setuid binary. Take the following scenario for
example: init script (as root) spawns a network manager program as uid
1000 and then the network manager spawns OpenVPN. The common mode of
operation for OpenVPN is to start running as the uid it was spawned
with (1000) at startup, but then drop to a lesser-privileged uid (e.g.
2000) after initialization/setup by calling setuid. This is something
setuid bits wouldn't help with, without refactoring OpenVPN. So one
option here is to give the network manager CAP_SETUID, which will be
inherited by OpenVPN, and then OpenVPN drops to uid 2000 and drops
CAP_SETUID (would probably require patching OpenVPN for the capability
dropping). The problem here is that if the network manager itself is
untrusted and exploitable, then giving it unrestricted CAP_SETUID is a
big security risk. Even just sticking with the network manager / VPN
example, strongSwan VPN also uses the same drop-to-user-through-setuid
setup, as do other Linux applications. Refactoring these applications
to fork/exec setuid binaries instead of simply calling setuid is often
infeasible. So a direct call to setuid is often necessary/expected,
and setuid bits don't help here.

Also, use of setuid bits precludes the use of the no_new_privs bit,
which is usually at least a nice-to-have (if not need-to-have) for
sandboxed processes on the system.

>
> >> I'm still not quite clear on whether you want N completely unprivileged
> >> uids to be used by some user (i.e. uid 1000), or whether one or more of
> >> those should also have some privileged, or whether one of the uids might
> >> or might not b uid 0.  Years ago I used to use N separate uids to
> >> somewhat segragate workloads on my laptop, and I'd like my browser to
> >> do something like that.  Is that the kind of uid switching you have
> >> in mind?
> > "N completely unprivileged uids to be used by some user (i.e. uid
> > 1000)" is the closest description of what this LSM has in mind. For
> > example, uid 123 is some system service that needs runtime
> > capabilities X, Y and Z and a bunch of DBus permissions associated
> > with uid 123, but also wants to spawn another program without any of
> > these capabilities/permissions. In this case we would like to avoid
> > giving the system service CAP_SETUID.
> >
> >> -serge
> >>
> >> *1 And maybe with one of the p9auth/factotem proposals out there you
> >> could have a userspace daemon hand out the tokens for setuid, but that's
> >> getting "out there" and probably derailing this conversation :)
>