linux-kernel - Re: [PATCH v10 09/12] ima: Implement support for module-style appended signatures

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Message-Id: <87y32q5qi2.fsf@morokweng.localdomain>
Date:   Tue, 28 May 2019 16:27:17 -0300
From:   Thiago Jung Bauermann <bauerman@...ux.ibm.com>
To:     Mimi Zohar <zohar@...ux.ibm.com>
Cc:     linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, keyrings@...r.kernel.org,
        linux-crypto@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        David Howells <dhowells@...hat.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Jessica Yu <jeyu@...nel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Jonathan Corbet <corbet@....net>,
        "AKASHI\, Takahiro" <takahiro.akashi@...aro.org>
Subject: Re: [PATCH v10 09/12] ima: Implement support for module-style appended signatures


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

> Hi Thiago,
>
> On Thu, 2019-04-18 at 00:51 -0300, Thiago Jung Bauermann wrote:
>> 
>> @@ -326,6 +356,10 @@ int ima_appraise_measurement(enum ima_hooks func,
>> case INTEGRITY_UNKNOWN:
>> break;
>> case INTEGRITY_NOXATTRS:/* No EVM protected xattrs. */
>> +/* It's fine not to have xattrs when using a modsig. */
>> +if (try_modsig)
>> +break;
>> +/* fall through */
>> case INTEGRITY_NOLABEL:/* No security.evm xattr. */
>> cause = "missing-HMAC";
>> goto out;
>> @@ -340,6 +374,14 @@ int ima_appraise_measurement(enum ima_hooks func,
>> rc = xattr_verify(func, iint, xattr_value, xattr_len, &status,
>>  &cause);
>> 
>> +/*
>> + * If we have a modsig and either no imasig or the imasig's key isn't
>> + * known, then try verifying the modsig.
>> + */
>> +if (status != INTEGRITY_PASS && try_modsig &&
>> + (!xattr_value || rc == -ENOKEY))
>> +rc = modsig_verify(func, modsig, &status, &cause);
>
> EVM protects other security xattrs, not just security.ima, if they
> exist. As a result, evm_verifyxattr() could pass based on the other
> security xattrs.

Indeed! It doesn't make sense to test for status != INTEGRITY_PASS here.
Not sure what I was thinking. Thanks for spotting it. With your other
comments about this if clause, this code now reads:

	/*
	 * If we have a modsig and either no imasig or the imasig's key isn't
	 * known, then try verifying the modsig.
	 */
	if (try_modsig &&
	    (!xattr_value || xattr_value->type == IMA_XATTR_DIGEST_NG ||
	     rc == -ENOKEY))
		rc = modsig_verify(func, modsig, &status, &cause);

-- 
Thiago Jung Bauermann
IBM Linux Technology Center