linux-kernel - Re: [PATCH v4 00/14] security: digest

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Date: Thu, 20 Jun 2024 17:14:28 +0200
From: Roberto Sassu <roberto.sassu@...weicloud.com>
To: Paul Moore <paul@...l-moore.com>
Cc: corbet@....net, jmorris@...ei.org, serge@...lyn.com, 
 akpm@...ux-foundation.org, shuah@...nel.org, mcoquelin.stm32@...il.com, 
 alexandre.torgue@...s.st.com, mic@...ikod.net, 
 linux-security-module@...r.kernel.org, linux-doc@...r.kernel.org, 
 linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, 
 bpf@...r.kernel.org, zohar@...ux.ibm.com, dmitry.kasatkin@...il.com, 
 linux-integrity@...r.kernel.org, wufan@...ux.microsoft.com,
 pbrobinson@...il.com,  zbyszek@...waw.pl, hch@....de, mjg59@...f.ucam.org,
 pmatilai@...hat.com,  jannh@...gle.com, dhowells@...hat.com,
 jikos@...nel.org, mkoutny@...e.com,  ppavlu@...e.com, petr.vorel@...il.com,
 mzerqung@...inter.de, kgold@...ux.ibm.com,  Roberto Sassu
 <roberto.sassu@...wei.com>
Subject: Re: [PATCH v4 00/14] security: digest_cache LSM

On Thu, 2024-06-20 at 10:48 -0400, Paul Moore wrote:
> On Thu, Jun 20, 2024 at 5:12 AM Roberto Sassu
> <roberto.sassu@...weicloud.com> wrote:
> > On Wed, 2024-06-19 at 14:43 -0400, Paul Moore wrote:
> > > On Wed, Jun 19, 2024 at 12:38 PM Roberto Sassu
> > > <roberto.sassu@...weicloud.com> wrote:
> > > > 
> > > > Making it a kernel subsystem would likely mean replicating what the LSM
> > > > infrastructure is doing, inode (security) blob and being notified about
> > > > file/directory changes.
> > > 
> > > Just because the LSM framework can be used for something, perhaps it
> > > even makes the implementation easier, it doesn't mean the framework
> > > should be used for everything.
> > 
> > It is supporting 3 LSMs: IMA, IPE and BPF LSM.
> > 
> > That makes it a clear target for the security subsystem, and as you
> > suggested to start for IMA, if other kernel subsystems require them, we
> > can make it as an independent subsystem.
> 
> Have you discussed the file digest cache functionality with either the
> IPE or BPF LSM maintainers?  While digest_cache may support these

Well, yes. I was in a discussion since long time ago with Deven and
Fan. The digest_cache LSM is listed in the Use Case section of the IPE
cover letter:

https://lore.kernel.org/linux-integrity/1716583609-21790-1-git-send-email-wufan@linux.microsoft.com/

I also developed an IPE module back in the DIGLIM days:

https://lore.kernel.org/linux-integrity/a16a628b9e21433198c490500a987121@huawei.com/

As for eBPF, I just need to make the digest_cache LSM API callable by
eBPF programs, very likely not requiring any change on the eBPF
infrastructure itself. As an example of the modification needed, you
could have a look at:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/trace/bpf_trace.c?h=v6.10-rc4#n1381


Once the digest_cache LSM API is exposed in eBPF, you could write a
simple file integrity check (taken from my DIGLIM eBPF), not tested:

SEC("lsm.s/bprm_creds_for_exec")
int BPF_PROG(exec, struct linux_binprm *bprm)
{
	u8 digest[MAX_DIGEST_SIZE] = { 0 };
	digest_cache_found_t found;
	struct digest_cache;
	int algo;

	algo = bpf_ima_file_hash(bprm->file, digest, sizeof(digest));
	if (algo < 0)
		return -EPERM;

	digest_cache = bpf_digest_cache_get(bprm->file->f_path.dentry);
	if (!digest_cache)
		return -EPERM;

	found = bpf_digest_cache_lookup(bprm->file->f_path.dentry,
					digest_cache, digest, algo);

	bpf_digest_cache_put(digest_cache);
	return found ? 0 : -EPERM;
}

Roberto

> LSMs, I don't recall seeing any comments from the other LSM
> developers; if you are going to advocate for this as something outside
> of IMA, it would be good to see a show of support for the other LSMs.
>