linux-kernel - Re: [PATCH v3 0/4] Introduce security_create_user

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Message-ID: <20220722061137.jahbjeucrljn2y45@kafai-mbp.dhcp.thefacebook.com>
Date:   Thu, 21 Jul 2022 23:11:37 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Frederick Lawler <fred@...udflare.com>
Cc:     kpsingh@...nel.org, revest@...omium.org, jackmanb@...omium.org,
        ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        songliubraving@...com, yhs@...com, john.fastabend@...il.com,
        jmorris@...ei.org, serge@...lyn.com, paul@...l-moore.com,
        stephen.smalley.work@...il.com, eparis@...isplace.org,
        shuah@...nel.org, brauner@...nel.org, casey@...aufler-ca.com,
        ebiederm@...ssion.com, bpf@...r.kernel.org,
        linux-security-module@...r.kernel.org, selinux@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, kernel-team@...udflare.com,
        cgzones@...glemail.com, karl@...badwolfsecurity.com
Subject: Re: [PATCH v3 0/4] Introduce security_create_user_ns()

On Thu, Jul 21, 2022 at 12:28:04PM -0500, 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 userns_create LSM hook, then marks the hook as sleepable in BPF.
Patch 1 and 4 still need review from the lsm/security side.