linux-kernel - Re: [RESEND][PATCH v4 2/4] bpf: Add bpf_request_key_by

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Message-ID: <CAADnVQL+hBsWaKJAVYT0OX4SZtVDO+rRZ-0OLq+3SaPQsGqLBg@mail.gmail.com>
Date:   Fri, 17 Jun 2022 10:06:49 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Roberto Sassu <roberto.sassu@...wei.com>
Cc:     "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "andrii@...nel.org" <andrii@...nel.org>,
        "kpsingh@...nel.org" <kpsingh@...nel.org>,
        "john.fastabend@...il.com" <john.fastabend@...il.com>,
        "songliubraving@...com" <songliubraving@...com>,
        "kafai@...com" <kafai@...com>, "yhs@...com" <yhs@...com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "keyrings@...r.kernel.org" <keyrings@...r.kernel.org>,
        "dhowells@...hat.com" <dhowells@...hat.com>
Subject: Re: [RESEND][PATCH v4 2/4] bpf: Add bpf_request_key_by_id() helper

On Fri, Jun 17, 2022 at 2:11 AM Roberto Sassu <roberto.sassu@...wei.com> wrote:
>
> > From: Alexei Starovoitov <alexei.starovoitov@...il.com>
> > Sent: Friday, June 17, 2022 5:46 AM
>
> Adding in CC the keyring mailing list and David.
>
> Sort summary: we are adding an eBPF helper, to let eBPF programs
> verify PKCS#7 signatures. The helper simply calls verify_pkcs7_signature().
>
> The problem is how to pass the key for verification.
>
> For hardcoded keyring IDs, it is easy, pass 0, 1 or 2 for respectively
> the built-in, secondary and platform keyring.
>
> If you want to pass another keyring, you need to do a lookup,
> which returns a key with reference count increased.
>
> While in the kernel you can call key_put() to decrease the
> reference count, that is not guaranteed with an eBPF program,
> if the developer forgets about it. What probably is necessary,
> is to add the capability to the verifier to check whether the
> reference count is decreased, or adding a callback mechanism
> to call automatically key_put() when the eBPF program is
> terminated.

Nothing special here.
See acquire/release logic in the verifier and relevant helpers.
Like bpf_sk_lookup_tcp and others.

> Is there an alternative solution?
>
> Thanks
>
> Roberto
>
> HUAWEI TECHNOLOGIES Duesseldorf GmbH,
> HRB 56063 Managing Director: Li Peng, Yang Xi, Li He

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