Date:   Tue, 5 Apr 2022 09:21:53 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Roberto Sassu <roberto.sassu@...wei.com>,
        Djalal Harouni <tixxdz@...il.com>,
        KP Singh <kpsingh@...nel.org>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        "corbet@....net" <corbet@....net>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "andrii@...nel.org" <andrii@...nel.org>,
        "shuah@...nel.org" <shuah@...nel.org>,
        "mcoquelin.stm32@...il.com" <mcoquelin.stm32@...il.com>,
        "alexandre.torgue@...s.st.com" <alexandre.torgue@...s.st.com>,
        "zohar@...ux.ibm.com" <zohar@...ux.ibm.com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "linux-stm32@...md-mailman.stormreply.com" 
        <linux-stm32@...md-mailman.stormreply.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [PATCH 00/18] bpf: Secure and authenticated preloading of eBPF
 programs

On 4/5/2022 8:29 AM, Roberto Sassu wrote:
>> From: Casey Schaufler [mailto:casey@...aufler-ca.com]
>> Sent: Tuesday, April 5, 2022 4:50 PM
>> On 4/4/2022 10:20 AM, Roberto Sassu wrote:
>>>> From: Djalal Harouni [mailto:tixxdz@...il.com]
>>>> Sent: Monday, April 4, 2022 9:45 AM
>>>> On Sun, Apr 3, 2022 at 5:42 PM KP Singh <kpsingh@...nel.org> wrote:
>>>>> On Sat, Apr 2, 2022 at 1:55 AM Alexei Starovoitov
>>>>> <alexei.starovoitov@...il.com> wrote:
>>>> ...
>>>>>>> Pinning
>>>>>>> them to unreachable inodes intuitively looked the
>>>>>>> way to go for achieving the stated goal.
>>>>>> We can consider inodes in bpffs that are not unlinkable by root
>>>>>> in the future, but certainly not for this use case.
>>>>> Can this not be already done by adding a BPF_LSM program to the
>>>>> inode_unlink LSM hook?
>>>>>
>>>> Also, beside of the inode_unlink... and out of curiosity: making
>> sysfs/bpffs/
>>>> readonly after pinning, then using bpf LSM hooks
>>>> sb_mount|remount|unmount...
>>>> family combining bpf() LSM hook... isn't this enough to:
>>>> 1. Restrict who can pin to bpffs without using a full MAC
>>>> 2. Restrict who can delete or unmount bpf filesystem
>>>>
>>>> ?
>>> I'm thinking to implement something like this.
>>>
>>> First, I add a new program flag called
>>> BPF_F_STOP_ONCONFIRM, which causes the ref count
>>> of the link to increase twice at creation time. In this way,
>>> user space cannot make the link disappear, unless a
>>> confirmation is explicitly sent via the bpf() system call.
>>>
>>> Another advantage is that other LSMs can decide
>>> whether or not they allow a program with this flag
>>> (in the bpf security hook).
>>>
>>> This would work regardless of the method used to
>>> load the eBPF program (user space or kernel space).
>>>
>>> Second, I extend the bpf() system call with a new
>>> subcommand, BPF_LINK_CONFIRM_STOP, which
>>> decreasres the ref count for the link of the programs
>>> with the BPF_F_STOP_ONCONFIRM flag. I will also
>>> introduce a new security hook (something like
>>> security_link_confirm_stop), so that an LSM has the
>>> opportunity to deny the stop (the bpf security hook
>>> would not be sufficient to determine exactly for
>>> which link the confirmation is given, an LSM should
>>> be able to deny the stop for its own programs).
>> Would you please stop referring to a set of eBPF programs
>> loaded into the BPF LSM as an LSM? Call it a BPF security
>> module (BSM) if you must use an abbreviation. An LSM is a
>> provider of security_ hooks. In your case that is BPF. When
>> you call the set of eBPF programs an LSM it is like calling
>> an SELinux policy an LSM.
> An eBPF program could be a provider of security_ hooks
> too.

No, it can't. If I look in /sys/kernel/security/lsm what
you see is "bpf". The LSM is BPF. What BPF does in its
hooks is up to it and its responsibility.

>   The bpf LSM is an aggregator, similarly to your
> infrastructure to manage built-in LSMs. Maybe, calling
> it second-level LSM or secondary LSM would better
> represent this new class.

It isn't an LSM, and adding a qualifier doesn't make it
one and only adds to the confusion.

> The only differences are the registration method, (SEC
> directive instead of DEFINE_LSM), and what the hook
> implementation can access.

Those two things pretty well define what an LSM is.

> The implementation of a security_ hook via eBPF can
> follow the same structure of built-in LSMs, i.e. it can be
> uniquely responsible for enforcing and be policy-agnostic,
> and can retrieve the decisions based on a policy from a
> component implemented somewhere else.

The BPF LSM provides mechanism. The eBPF programs provide policy.

>
> Hopefully, I understood the basic principles correctly.
> I let the eBPF maintainers comment on this.
>
> Thanks
>
> Roberto
>
> HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
> Managing Director: Li Peng, Zhong Ronghua
>
>>> What do you think?
>>>
>>> Thanks
>>>
>>> Roberto
>>>
>>> HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
>>> Managing Director: Li Peng, Zhong Ronghua

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