linux-kernel - Re: [PATCH v3 06/11] evm: Ignore INTEGRITY

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Date:   Fri, 04 Dec 2020 08:04:57 -0500
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Roberto Sassu <roberto.sassu@...wei.com>,
        "mjg59@...gle.com" <mjg59@...gle.com>
Cc:     "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Silviu Vlasceanu <Silviu.Vlasceanu@...wei.com>
Subject: Re: [PATCH v3 06/11] evm: Ignore INTEGRITY_NOLABEL if no HMAC key
 is loaded

On Fri, 2020-12-04 at 08:05 +0000, Roberto Sassu wrote:
> > From: Mimi Zohar [mailto:zohar@...ux.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.

Right, there are a couple of  scenarios.  Let's be more specific as to
which scenario this patch is addressing.

- a public key is loaded and EVM_ALLOW_METADATA_WRITES is enabled,
- a public key is loaded and EVM_ALLOW_METADATA_WRITES is disabled,
- an HMAC key is loaded

For the first and last case, this patch shouldn't be necessary.  Only
the second case, with EVM_ALLOW_METADATA_WRITES disabled, probably does
not work.  I would claim that is working as designed.

thanks,

Mimi