linux-kernel - Re: [PATCH 0/2] support to read and tune appraise mode in runtime

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Date:   Mon, 13 Apr 2020 17:55:58 -0400
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Tianjia Zhang <tianjia.zhang@...ux.alibaba.com>,
        dmitry.kasatkin@...il.com, jmorris@...ei.org, serge@...lyn.com,
        zhangliguang@...ux.alibaba.com
Cc:     linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, Nayna Jain <nayna@...ux.ibm.com>
Subject: Re: [PATCH 0/2] support to read and tune appraise mode in runtime

On Thu, 2020-04-09 at 11:39 +0800, Tianjia Zhang wrote:
> Support the read and write operations of ima_appraise by adding a
> securifyfs file 'appraise_mode'.
> 
> In order to tune appraise mode in runtime, writing a PKCS#7 signature
> corresponding the signed content is required. The content should be off,
> enforce, log or fix. Given a simple way to archive this:
> 
> $ echo -n off > mode
> $ openssl smime -sign -nocerts -noattr -binary \
>     -in mode -inkey <system_trusted_key> \
>     -signer <cert> -outform der -out mode.p7s
> $ sudo cat mode.p7s \
>     > /sys/kernel/security/ima/appraise_mode
> 
> Note that the signing key must be a trust key located in
> system trusted keyring. So even the root privilege cannot
> simply disable the enforcement.

There are major problems with disabling IMA appraisal.  This patch set
proposes disabling IMA appraisal without even providing the motivation
for such support.

A lot of effort went into preventing custom IMA policies from
disabling appraising the kexec or kernel module signatures.  In
addition, the "lockdown" patch set was upstreamed permitting IMA
signature verification.  This patch set would break both of these
features.

IMA relies on its own keyring for verifying file signatures, not the
builtin or secondary trusted kernel keyrings.

Two methods already exist - xattr and appended signatures - for
verifying file signatures.  This patch set assumes creating and
signing a file, which is then written to a securityfs file.  Like for
loading a custom IMA policy, instead of cat'ing the file, write the
pathname to the securityfs file.

If you must define a new IMA method for verifying file signatures,
then it needs to be generic and added to ima_appraise_measurement().
 (Refer to the new IMA appended signature support.)

Mimi

> 
> Tianjia Zhang (2):
>   ima: support to read appraise mode
>   ima: support to tune appraise mode in runtime
> 
>  security/integrity/ima/ima_fs.c | 134 +++++++++++++++++++++++++++++++-
>  1 file changed, 133 insertions(+), 1 deletion(-)
>