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

[Micah Morton]

On Fri, Nov 2, 2018 at 9:05 AM Casey Schaufler <casey at schaufler-ca.com> wrote:
>
> On 11/1/2018 12:52 PM, Micah Morton wrote:
> > 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.
>
> Yes, I understand that.
>
> > Take the following scenario for
> > example: init script (as root) spawns a network manager program as uid
> > 1000
>
> So far, so good.
>
> > 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.
>
> OK. That's an operation that does and ought to require privilege.

Sure, but the idea behind this LSM is that full CAP_SETUID
capabilities are a lot more privilege than is necessary in this
scenario.

>
> > This is something
> > setuid bits wouldn't help with, without refactoring OpenVPN.
>
> You're correct.
>
> > 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).
>
> Or, you put CAP_SETUID on the file capabilities for OpenVPN,
> which is the way the P1003.1e DRAFT specification would have
> you accomplish this. Unfortunately, with all the changes made
> to capabilities for namespaces and all I'm not 100% sure I
> could say exactly how to set that.
>
> > 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.
>
> Right. That's why you set the file capabilities on OpenVPN.

So it seems like you're suggesting that any time a program needs to
switch user by calling setuid, that it should get full CAP_SETUID
capabilities (whether that's through setting file capabilities on the
binary or inheriting CAP_SETUID from a parent process or otherwise).
But that brings us back to the basic problem this LSM is trying to
solve. Namely, we don't want to sprinkle unrestricted CAP_SETUID privs
all over the system for binaries that just want to switch to specific
uid[s] and don't need any of the root-equivalent privileges provided
by CAP_SETUID.

>
> > 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.
>
> Same solution.
>
> > 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.
>
> What is it with kids these days, that they are so
> afraid of fixing code that needs fixing? But that's
> not necessary in this example.
>
> > 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.
>
> But you've already said that you *want* to change the security state,
> "drop to a lesser-privileged uid", so you're already mucking with the
> sandbox. If you're going to say that changing UIDs doesn't count for
> sandboxing I'll point out that you brought up the notion of a
> lesser-privileged UID.

There are plenty of ways that non-root processes further restrict
especially vulnerable parts of their code to even lesser-privileged
contexts. But its often easier to reason about the security of such
applications if the no_new_privs bit is set and file capabilities are
avoided, so the application can have full control of which privileges
are given to spawned processes without having to worry about which
privileges are attached to which files. Granted, the no_new_privs
issue is less central to the LSM being proposed here compared to the
discussion above.

>