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Message-ID: <3dbd5b30-f869-b284-1383-309ca6994557@cloudflare.com>
Date: Fri, 8 Jul 2022 09:01:32 -0500
From: Frederick Lawler <fred@...udflare.com>
To: Christian Göttsche <cgzones@...glemail.com>
Cc: KP Singh <kpsingh@...nel.org>, revest@...omium.org,
jackmanb@...omium.org, Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>,
Martin KaFai Lau <kafai@...com>,
Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
John Fastabend <john.fastabend@...il.com>,
James Morris <jmorris@...ei.org>,
"Serge E. Hallyn" <serge@...lyn.com>,
Paul Moore <paul@...l-moore.com>,
Stephen Smalley <stephen.smalley.work@...il.com>,
Eric Paris <eparis@...isplace.org>, shuah@...nel.org,
Christian Brauner <brauner@...nel.org>,
Casey Schaufler <casey@...aufler-ca.com>,
"Eric W. Biederman" <ebiederm@...ssion.com>, bpf@...r.kernel.org,
linux-security-module@...r.kernel.org,
SElinux list <selinux@...r.kernel.org>,
linux-kselftest@...r.kernel.org,
Linux kernel mailing list <linux-kernel@...r.kernel.org>,
netdev@...r.kernel.org, kernel-team@...udflare.com
Subject: Re: [PATCH v2 0/4] Introduce security_create_user_ns()
On 7/8/22 7:10 AM, Christian Göttsche wrote:
> ,On Fri, 8 Jul 2022 at 00:32, Frederick Lawler <fred@...udflare.com> 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?
> 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;
>
Links:
1.
https://lore.kernel.org/all/CAHC9VhSTkEMT90Tk+=iTyp3npWEm+3imrkFVX2qb=XsOPp9F=A@mail.gmail.com/
>>
>> Links:
>> 1. https://lore.kernel.org/all/20220608150942.776446-1-fred@cloudflare.com/
>> 2. https://lore.kernel.org/all/87y1xzyhub.fsf@email.froward.int.ebiederm.org/
>> 3. https://lore.kernel.org/all/9fe9cd9f-1ded-a179-8ded-5fde8960a586@cloudflare.com/
>>
>> 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
>>
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