RE: [PATCH 7/8] ima: use ima_hash_algo for collision detection in the measurement list

[Roberto Sassu]

> -----Original Message-----
> From: linux-integrity-owner@xxxxxxxxxxxxxxx [mailto:linux-integrity-
> owner@xxxxxxxxxxxxxxx] On Behalf Of Mimi Zohar
> Sent: Thursday, January 30, 2020 11:26 PM
> To: Roberto Sassu <roberto.sassu@xxxxxxxxxx>;
> jarkko.sakkinen@xxxxxxxxxxxxxxx;
> james.bottomley@xxxxxxxxxxxxxxxxxxxxx; linux-integrity@xxxxxxxxxxxxxxx
> Cc: linux-security-module@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> Silviu Vlasceanu <Silviu.Vlasceanu@xxxxxxxxxx>
> Subject: Re: [PATCH 7/8] ima: use ima_hash_algo for collision detection in
> the measurement list
> 
> On Mon, 2020-01-27 at 18:04 +0100, Roberto Sassu wrote:
> > Before calculating a digest for each PCR bank, collisions were detected
> > with a SHA1 digest. This patch includes ima_hash_algo among the
> algorithms
> > used to calculate the template digest and checks collisions on that digest.
> >
> > Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> 
> Definitely needed to protect against a sha1 collision attack.
> 
> <snip>
> 
> >
> > diff --git a/security/integrity/ima/ima_api.c
> b/security/integrity/ima/ima_api.c
> > index ebaf0056735c..a9bb45de6db9 100644
> > --- a/security/integrity/ima/ima_api.c
> > +++ b/security/integrity/ima/ima_api.c
> > @@ -51,7 +51,7 @@ int ima_alloc_init_template(struct ima_event_data
> *event_data,
> >  	if (!*entry)
> >  		return -ENOMEM;
> >
> > -	(*entry)->digests = kcalloc(ima_tpm_chip->nr_allocated_banks + 1,
> > +	(*entry)->digests = kcalloc(ima_tpm_chip->nr_allocated_banks + 2,
> >  				    sizeof(*(*entry)->digests), GFP_NOFS);
> >  	if (!(*entry)->digests) {
> >  		result = -ENOMEM;
> 
> I would prefer not having to allocate and use "nr_allocated_banks + 1"
> everywhere, but I understand the need for it. ÂI'm not sure this patch
> warrants allocating +2. ÂPerhaps, if a TPM bank doesn't exist for the
> IMA default hash algorithm, use a different algorithm or, worst case,
> continue using the ima_sha1_idx.

We could introduce a new option called ima_hash_algo_tpm to specify
the algorithm of an allocated bank. We can use this for boot_aggregate
and hash collision detection.

Roberto

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