Re: [PATCH v2 0/4] Introduce security_create_user_ns()
From: Frederick Lawler
Date: Tue Jul 19 2022 - 16:00:30 EST
On 7/14/22 9:27 AM, Serge E. Hallyn wrote:
On Fri, Jul 08, 2022 at 09:11:15AM -0700, Casey Schaufler wrote:
On 7/8/2022 7:01 AM, Frederick Lawler wrote:
On 7/8/22 7:10 AM, Christian Göttsche wrote:
,On Fri, 8 Jul 2022 at 00:32, Frederick Lawler <fred@xxxxxxxxxxxxxx>
wrote:
While creating a LSM BPF MAC policy to block user namespace
creation, we
used the LSM cred_prepare hook because that is the closest hook to
prevent
a call to create_user_ns().
The calls look something like this:
cred = prepare_creds()
security_prepare_creds()
call_int_hook(cred_prepare, ...
if (cred)
create_user_ns(cred)
We noticed that error codes were not propagated from this hook and
introduced a patch [1] to propagate those errors.
The discussion notes that security_prepare_creds()
is not appropriate for MAC policies, and instead the hook is
meant for LSM authors to prepare credentials for mutation. [2]
Ultimately, we concluded that a better course of action is to introduce
a new security hook for LSM authors. [3]
This patch set first introduces a new security_create_user_ns()
function
and create_user_ns LSM hook, then marks the hook as sleepable in BPF.
Some thoughts:
I.
Why not make the hook more generic, e.g. support all other existing
and potential future namespaces?
The main issue with a generic hook is that different namespaces have
different calling contexts. We decided in a previous discussion to
opt-out of a generic hook for this reason. [1]
Also I think the naming scheme is <object>_<verb>.
That's a good call out. I was originally hoping to keep the
security_*() match with the hook name matched with the caller function
to keep things all aligned. If no one objects to renaming the hook, I
can rename the hook for v3.
LSM_HOOK(int, 0, namespace_create, const struct cred *cred,
unsigned int flags)
where flags is a bitmap of CLONE flags from include/uapi/linux/sched.h
(like CLONE_NEWUSER).
II.
While adding policing for namespaces maybe also add a new hook for
setns(2)
LSM_HOOK(int, 0, namespace_join, const struct cred *subj, const
struct cred *obj, unsigned int flags)
IIUC, setns() will create a new namespace for the other namespaces
except for user namespace. If we add a security hook for the other
create_*_ns() functions, then we can catch setns() at that point.
III.
Maybe even attach a security context to namespaces so they can be
further governed?
That would likely add confusion to the existing security module namespace
efforts. SELinux, Smack and AppArmor have all developed namespace models.
That, or it could replace the various independent efforts with a single,
I feel like you're attaching more meaning to this than there needs to be.
I *think* he's just talking about a user_namespace->u_security void*.
So that for instance while deciding whether to allow some transition,
selinux could check whether the caller's user namespace was created by
a task in an selinux context authorized to create user namespaces.
The "user namespaces are DAC and orthogonal to MAC" is of course true
(where the LSM does not itself tie them together), except that we all
know that a process running as root in a user namespace gains access to
often-less-trustworthy code gated under CAP_SYS_ADMIN.
unified security module namespace effort. There's more work to that than
adding a context to a namespace. Treating namespaces as objects is almost,
but not quite, solidifying containers as a kernel construct. We know we
can't do that.
What we "can't do" (imo) is to create a "full container" construct which
ties together the various namespaces and other concepts in a restrictive
way.
Is this the direction we want to go with the SELinux implementation? If
so, where can I find a similar implementation to make the userns_create
work with this? If not, I have a v3 with the hook name change ready to post.
SELinux example:
type domainA_userns_t;
type_transition domainA_t domainA_t : namespace domainA_userns_t
"user";
allow domainA_t domainA_userns_t:namespace create;
# domainB calling setns(2) with domainA as target
allow domainB_t domainA_userns_t:namespace join;
While I'm not an expert on SELinux policy, I'd bet a refreshing beverage
that there's already a way to achieve this with existing constructs.
Smack, which is subject+object MAC couldn't care less about the user
namespace configuration. User namespaces are DAC constructs.
Links:
1.
https://lore.kernel.org/all/CAHC9VhSTkEMT90Tk+=iTyp3npWEm+3imrkFVX2qb=XsOPp9F=A@xxxxxxxxxxxxxx/
Links:
1.
https://lore.kernel.org/all/20220608150942.776446-1-fred@xxxxxxxxxxxxxx/
2.
https://lore.kernel.org/all/87y1xzyhub.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
3.
https://lore.kernel.org/all/9fe9cd9f-1ded-a179-8ded-5fde8960a586@xxxxxxxxxxxxxx/
Changes since v1:
- Add selftests/bpf: Add tests verifying bpf lsm create_user_ns hook
patch
- Add selinux: Implement create_user_ns hook patch
- Change function signature of security_create_user_ns() to only take
struct cred
- Move security_create_user_ns() call after id mapping check in
create_user_ns()
- Update documentation to reflect changes
Frederick Lawler (4):
security, lsm: Introduce security_create_user_ns()
bpf-lsm: Make bpf_lsm_create_user_ns() sleepable
selftests/bpf: Add tests verifying bpf lsm create_user_ns hook
selinux: Implement create_user_ns hook
include/linux/lsm_hook_defs.h | 1 +
include/linux/lsm_hooks.h | 4 +
include/linux/security.h | 6 ++
kernel/bpf/bpf_lsm.c | 1 +
kernel/user_namespace.c | 5 ++
security/security.c | 5 ++
security/selinux/hooks.c | 9 ++
security/selinux/include/classmap.h | 2 +
.../selftests/bpf/prog_tests/deny_namespace.c | 88
+++++++++++++++++++
.../selftests/bpf/progs/test_deny_namespace.c | 39 ++++++++
10 files changed, 160 insertions(+)
create mode 100644
tools/testing/selftests/bpf/prog_tests/deny_namespace.c
create mode 100644
tools/testing/selftests/bpf/progs/test_deny_namespace.c
--
2.30.2