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Subject: Re: [PATCH v3 2/2] ima: add enforce-evm and log-evm modes to
 strictly check EVM status
From:   Mimi Zohar <zohar@linux.ibm.com>
To:     Roberto Sassu <roberto.sassu@huawei.com>,
        dmitry.kasatkin@huawei.com, mjg59@google.com
Cc:     linux-integrity@vger.kernel.org,
        linux-security-module@vger.kernel.org, linux-doc@vger.kernel.org,
        stable@vger.kernel.org, linux-kernel@vger.kernel.org,
        silviu.vlasceanu@huawei.com
Date:   Fri, 07 Jun 2019 11:08:32 -0400
In-Reply-To: <93459fe8-f9b6-fe45-1ca7-2efb8854dc8b@huawei.com>
References: <20190606112620.26488-1-roberto.sassu@huawei.com>
         <20190606112620.26488-3-roberto.sassu@huawei.com>
         <1559917462.4278.253.camel@linux.ibm.com>
         <93459fe8-f9b6-fe45-1ca7-2efb8854dc8b@huawei.com>
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On Fri, 2019-06-07 at 16:40 +0200, Roberto Sassu wrote:
> > On Thu, 2019-06-06 at 13:26 +0200, Roberto Sassu wrote:

> >> Although this choice appears legitimate, it might not be suitable for
> >> hardened systems, where the administrator expects that access is denied if
> >> there is any error. An attacker could intentionally delete the EVM keys
> >> from the system and set the file digest in security.ima to the actual file
> >> digest so that the final appraisal status is INTEGRITY_PASS.
> > 
> > Assuming that the EVM HMAC key is stored in the initramfs, not on some
> > other file system, and the initramfs is signed, INTEGRITY_UNKNOWN
> > would be limited to the rootfs filesystem.
> 
> There is another issue. The HMAC key, like the public keys, should be
> loaded when appraisal is disabled. This means that we have to create a
> trusted key at early boot and defer the unsealing.

There is no need for IMA to appraise the public key file signature,
since the certificate is signed by a key on the builtin/secondary
trusted keyring.  With CONFIG_IMA_LOAD_X509 enabled, the public key
can be loaded onto the IMA keyring with IMA-appraisal enabled, but
without verifying the file signature.

Mimi