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Subject: Re: [PATCH v10 09/12] ima: Implement support for module-style
 appended signatures
From: Mimi Zohar <zohar@linux.ibm.com>
To: Thiago Jung Bauermann <bauerman@linux.ibm.com>,
 linux-integrity@vger.kernel.org
Date: Tue, 14 May 2019 08:09:25 -0400
In-Reply-To: <20190418035120.2354-10-bauerman@linux.ibm.com>
References: <20190418035120.2354-1-bauerman@linux.ibm.com>
 <20190418035120.2354-10-bauerman@linux.ibm.com>
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Cc: Herbert Xu <herbert@gondor.apana.org.au>, linux-doc@vger.kernel.org,
 Dmitry Kasatkin <dmitry.kasatkin@gmail.com>,
 "David S. Miller" <davem@davemloft.net>, Jonathan Corbet <corbet@lwn.net>,
 linux-kernel@vger.kernel.org, James Morris <jmorris@namei.org>,
 David Howells <dhowells@redhat.com>, "AKASHI,
 Takahiro" <takahiro.akashi@linaro.org>, linux-security-module@vger.kernel.org,
 keyrings@vger.kernel.org, linux-crypto@vger.kernel.org,
 Jessica Yu <jeyu@kernel.org>, linuxppc-dev@lists.ozlabs.org,
 David Woodhouse <dwmw2@infradead.org>, "Serge E. Hallyn" <serge@hallyn.com>
Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org
Sender: "Linuxppc-dev"
 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

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.

Mimi

> +
>  out:
>         /*
>          * File signatures on some filesystems can not be properly verified.