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From:   Roberto Sassu <roberto.sassu@huawei.com>
To:     Mimi Zohar <zohar@linux.ibm.com>,
        "mjg59@google.com" <mjg59@google.com>
CC:     "linux-integrity@vger.kernel.org" <linux-integrity@vger.kernel.org>,
        "linux-security-module@vger.kernel.org" 
        <linux-security-module@vger.kernel.org>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Silviu Vlasceanu <Silviu.Vlasceanu@huawei.com>
Subject: RE: [PATCH v3 06/11] evm: Ignore INTEGRITY_NOLABEL if no HMAC key is
 loaded
Thread-Topic: [PATCH v3 06/11] evm: Ignore INTEGRITY_NOLABEL if no HMAC key is
 loaded
Thread-Index: AQHWuAyPc3OXOTvUVE6QYcHvma97kKnl6NcAgADOOtA=
Date:   Fri, 4 Dec 2020 08:05:05 +0000
Message-ID: <3c628dc54804469597a72d03c33e8315@huawei.com>
References: <20201111092302.1589-1-roberto.sassu@huawei.com>
         <20201111092302.1589-7-roberto.sassu@huawei.com>
 <b9f1a31e9b2dfb7a7167574a39652932263488e8.camel@linux.ibm.com>
In-Reply-To: <b9f1a31e9b2dfb7a7167574a39652932263488e8.camel@linux.ibm.com>
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> From: Mimi Zohar [mailto:zohar@linux.ibm.com]
> Sent: Thursday, December 3, 2020 9:43 PM
> Hi Roberto,
> 
> On Wed, 2020-11-11 at 10:22 +0100, Roberto Sassu wrote:
> > When a file is being created, LSMs can set the initial label with the
> > inode_init_security hook. If no HMAC key is loaded, the new file will have
> > LSM xattrs but not the HMAC.
> >
> > Unfortunately, EVM will deny any further metadata operation on new
> files,
> > as evm_protect_xattr() will always return the INTEGRITY_NOLABEL error.
> This
> > would limit the usability of EVM when only a public key is loaded, as
> > commands such as cp or tar with the option to preserve xattrs won't work.
> >
> > Ignoring this error won't be an issue if no HMAC key is loaded, as the
> > inode is locked until the post hook, and EVM won't calculate the HMAC on
> > metadata that wasn't previously verified. Thus this patch checks if an
> > HMAC key is loaded and if not, ignores INTEGRITY_NOLABEL.
> 
> I'm not sure what problem this patch is trying to solve.
> evm_protect_xattr() is only called by evm_inode_setxattr() and
> evm_inode_removexattr(), which first checks whether
> EVM_ALLOW_METADATA_WRITES is enabled.

The idea is to also support EVM verification when only a public key
is loaded. An advantage to do that is that for example we can prevent
accidental metadata changes when the signature is portable.

Roberto

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