linux-kernel - Re: [PATCH 1/3] bpf: Add bpf_check

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Message-ID: <aDgy1Wqn7WIFNXvb@wunner.de>
Date: Thu, 29 May 2025 12:11:33 +0200
From: Lukas Wunner <lukas@...ner.de>
To: Blaise Boscaccy <bboscaccy@...ux.microsoft.com>
Cc: Paul Moore <paul@...l-moore.com>, jarkko@...nel.org,
	zeffron@...tgames.com, xiyou.wangcong@...il.com,
	kysrinivasan@...il.com, code@...icks.com,
	linux-security-module@...r.kernel.org, roberto.sassu@...wei.com,
	James.Bottomley@...senpartnership.com,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	John Fastabend <john.fastabend@...il.com>,
	Andrii Nakryiko <andrii@...nel.org>,
	Martin KaFai Lau <martin.lau@...ux.dev>,
	Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>,
	Yonghong Song <yonghong.song@...ux.dev>,
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>,
	Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
	David Howells <dhowells@...hat.com>,
	Ignat Korchagin <ignat@...udflare.com>,
	Quentin Monnet <qmo@...nel.org>,
	Jason Xing <kerneljasonxing@...il.com>,
	Willem de Bruijn <willemb@...gle.com>,
	Anton Protopopov <aspsk@...valent.com>,
	Jordan Rome <linux@...danrome.com>,
	Martin Kelly <martin.kelly@...wdstrike.com>,
	Alan Maguire <alan.maguire@...cle.com>,
	Matteo Croce <teknoraver@...a.com>, bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org, keyrings@...r.kernel.org,
	linux-crypto@...r.kernel.org
Subject: Re: [PATCH 1/3] bpf: Add bpf_check_signature

On Wed, May 28, 2025 at 02:49:03PM -0700, Blaise Boscaccy wrote:
> +	if (!attr->signature_maps_size) {
> +		sha256((u8 *)prog->insnsi, prog->len * sizeof(struct bpf_insn), (u8 *)&hash);
> +		err = verify_pkcs7_signature(hash, sizeof(hash), signature, attr->signature_size,
> +				     VERIFY_USE_SECONDARY_KEYRING,
> +				     VERIFYING_EBPF_SIGNATURE,
> +				     NULL, NULL);

Has this ever been tested?

It looks like it will always return -EINVAL because:

  verify_pkcs7_signature()
    verify_pkcs7_message_sig()
      pkcs7_verify()

... pkcs7_verify() contains a switch statement which you're not
amending with a "case VERIFYING_EBPF_SIGNATURE" but which returns
-EINVAL in the "default" case.

Aside from that, you may want to consider introducing a new ".ebpf"
keyring to allow adding trusted keys specifically for eBPF verification
without having to rely on the system keyring.

Constraining oneself to sha256 doesn't seem future-proof.

Some minor style issues in the commit message caught my eye:

> This introduces signature verification for eBPF programs inside of the
> bpf subsystem. Two signature validation schemes are included, one that

Use imperative mood, avoid repetitive "This ...", e.g.
"Introduce signature verification of eBPF programs..."

> The signature check is performed before the call to
> security_bpf_prog_load. This allows the LSM subsystem to be clued into
> the result of the signature check, whilst granting knowledge of the
> method and apparatus which was employed.

"Perform the signature check before calling security_bpf_prog_load()
to allow..."

Thanks,

Lukas