linux-kernel - Re: [RFC PATCH v2 03/13] ima: invalidate unsupported PCR banks

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Message-ID: <87semzoocb.fsf@>
Date: Wed, 26 Mar 2025 15:31:32 +0100
From: Nicolai Stange <nstange@...e.de>
To: Mimi Zohar <zohar@...ux.ibm.com>
Cc: Nicolai Stange <nstange@...e.de>,  Roberto Sassu
 <roberto.sassu@...wei.com>,  Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
  Eric Snowberg <eric.snowberg@...cle.com>,  Jarkko Sakkinen
 <jarkko@...nel.org>,  James Bottomley
 <James.Bottomley@...senPartnership.com>,  linux-integrity@...r.kernel.org,
  linux-security-module@...r.kernel.org,  linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 03/13] ima: invalidate unsupported PCR banks

Mimi Zohar <zohar@...ux.ibm.com> writes:

> On Wed, 2025-03-26 at 10:01 +0100, Nicolai Stange wrote:
>> Mimi Zohar <zohar@...ux.ibm.com> writes:
>> 
>> > > diff --git a/security/integrity/ima/ima_crypto.c b/security/integrity/ima/ima_crypto.c
>> > > index 6f5696d999d0..a43080fb8edc 100644
>> > > --- a/security/integrity/ima/ima_crypto.c
>> > > +++ b/security/integrity/ima/ima_crypto.c
>> > > @@ -625,26 +625,43 @@ int ima_calc_field_array_hash(struct ima_field_data *field_data,
>> > >  	u16 alg_id;
>> > >  	int rc, i;
>> > >  
>> > > +#if IS_ENABLED(CONFIG_IMA_COMPAT_FALLBACK_TPM_EXTEND)
>> > >  	rc = ima_calc_field_array_hash_tfm(field_data, entry, ima_sha1_idx);
>> > >  	if (rc)
>> > >  		return rc;
>> > >  
>> > >  	entry->digests[ima_sha1_idx].alg_id = TPM_ALG_SHA1;
>> > > +#endif
>> > >  
>> > >  	for (i = 0; i < NR_BANKS(ima_tpm_chip) + ima_extra_slots; i++) {
>> > > +#if IS_ENABLED(CONFIG_IMA_COMPAT_FALLBACK_TPM_EXTEND)
>> > >  		if (i == ima_sha1_idx)
>> > >  			continue;
>> > > +#endif
>> > >  
>> > >  		if (i < NR_BANKS(ima_tpm_chip)) {
>> > >  			alg_id = ima_tpm_chip->allocated_banks[i].alg_id;
>> > >  			entry->digests[i].alg_id = alg_id;
>> > >  		}
>> > >  
>> > > -		/* for unmapped TPM algorithms digest is still a padded SHA1 */
>> > > +		/*
>> > > +		 * For unmapped TPM algorithms, the digest is still a
>> > > +		 * padded SHA1 if backwards-compatibility fallback PCR
>> > > +		 * extension is enabled. Otherwise fill with
>> > > +		 * 0xfes. This is the value to invalidate unsupported
>> > > +		 * PCR banks with. Also, a non-all-zeroes value serves
>> > > +		 * as an indicator to kexec measurement restoration
>> > > +		 * that the entry is not a violation and all its
>> > > +		 * template digests need to get recomputed.
>> > > +		 */
>> > >  		if (!ima_algo_array[i].tfm) {
>> > > +#if IS_ENABLED(CONFIG_IMA_COMPAT_FALLBACK_TPM_EXTEND)
>> > >  			memcpy(entry->digests[i].digest,
>> > >  			       entry->digests[ima_sha1_idx].digest,
>> > >  			       TPM_DIGEST_SIZE);
>> 
>>                                ^
>> That's been here before, just for the record for the below.
>
> And it is correct.
>
>> 
>> > > +#else
>> > > +			memset(entry->digests[i].digest, 0xfe, TPM_DIGEST_SIZE);
>> > > +#endif
>> > 
>> > Using TPM_DIGEST_SIZE will result in a padded 0xfe value.
>> 
>> Yes, but as the sysfs files for unsupported algos are gone, this will be
>> used only for extending the PCR banks. tpm[12]_pcr_extend()
>> (necessarily) truncate the digests to the correct size before sending
>> them to the TPM.
>> 
>> But if you prefer I can absolutely replace TPM_DIGEST_SIZE by
>> hash_digest_size[ima_algo_array[i].algo].
>
> Unlike violations, which are the full digest size, a padded sha1 is extended
> into the unsupported algos TPM banks.  I assume you'd want it to be the full
> digest size like violations.

You are so right, thanks for spotting! What I wanted is
TPM_MAX_DIGEST_SIZE, not TPM_DIGEST_SIZE (== 20).

Thanks,

Nicolai

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