linux-kernel - Re: [RFC][PATCH 8/8] ima: Detect if digest cache changed since last measurement/appraisal

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Date: Mon, 11 Mar 2024 08:19:56 -0400
From: Mimi Zohar <zohar@...ux.ibm.com>
To: Roberto Sassu <roberto.sassu@...weicloud.com>, corbet@....net,
        dmitry.kasatkin@...il.com, eric.snowberg@...cle.com,
        paul@...l-moore.com, jmorris@...ei.org, serge@...lyn.com
Cc: linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-integrity@...r.kernel.org, linux-security-module@...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, petrtesarik@...weicloud.com,
        mzerqung@...inter.de, kgold@...ux.ibm.com,
        Roberto Sassu <roberto.sassu@...wei.com>
Subject: Re: [RFC][PATCH 8/8] ima: Detect if digest cache changed since last
 measurement/appraisal

On Mon, 2024-03-11 at 10:11 +0100, Roberto Sassu wrote:
> 
> > > @@ -386,8 +402,6 @@ static int process_measurement(struct file *file,
> > > const
> > > struct cred *cred,
> > >  			if (verif_mask_ptr)
> > >  				allow_mask = policy_mask & *verif_mask_ptr;
> > >  		}
> > > -
> > > -		digest_cache_put(digest_cache);
> > 
> > Keeping a reference to the digest_cache list for each file in the iint cache
> > until the file is re-accessed, might take a while to free.
> 
> Yes, that is the drawback...
> 
> > I'm wondering if it necessary to keep a reference to the digest_cache.  Or
> > is it
> > possible to just compare the existing iint->digest_cache pointer with the
> > current digest_cache pointer?
> 
> If the pointer value is the same, it does not guarantee that it is the
> same digest cache used for the previous verification. It might have
> been freed and reallocated.

Agreed.
> 
> Maybe, if the digest_cache LSM is able to notify to IMA that the digest
> cache changed, so that IMA resets its flags in the integrity metadata,
> we would not need to pin it.

Yes, something similar to the "ima_lsm_policy_notifier".

Mimi