linux-kernel - Re: [PATCH 0/2] Introduce security_create_user

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Date:   Tue, 21 Jun 2022 17:19:15 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Frederick Lawler <fred@...udflare.com>, kpsingh@...nel.org,
        revest@...omium.org, jackmanb@...omium.org, ast@...nel.org,
        daniel@...earbox.net, andrii@...nel.org, kafai@...com,
        songliubraving@...com, yhs@...com, john.fastabend@...il.com,
        jmorris@...ei.org, serge@...lyn.com, bpf@...r.kernel.org,
        linux-security-module@...r.kernel.org
Cc:     brauner@...nel.org, paul@...l-moore.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...udflare.com,
        Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [PATCH 0/2] Introduce security_create_user_ns()

On 6/21/2022 4:39 PM, Frederick Lawler 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.

Why restrict this hook to user namespaces? It seems that an LSM that
chooses to preform controls on user namespaces may want to do so for
network namespaces as well. Also, the hook seems backwards. You should
decide if the creation of the namespace is allowed before you create it.
Passing the new namespace to a function that checks to see creating a
namespace is allowed doesn't make a lot off sense.

>
> 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/
>
> Frederick Lawler (2):
>    security, lsm: Introduce security_create_user_ns()
>    bpf-lsm: Make bpf_lsm_create_user_ns() sleepable
>
>   include/linux/lsm_hook_defs.h | 2 ++
>   include/linux/lsm_hooks.h     | 5 +++++
>   include/linux/security.h      | 8 ++++++++
>   kernel/bpf/bpf_lsm.c          | 1 +
>   kernel/user_namespace.c       | 5 +++++
>   security/security.c           | 6 ++++++
>   6 files changed, 27 insertions(+)
>
> --
> 2.30.2
>