linux-kernel - Re: [PATCH v9 2/8] powerpc/ima: add support to initialize ima policy rules

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Date:   Mon, 28 Oct 2019 07:54:56 -0400
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Nayna Jain <nayna@...ux.vnet.ibm.com>,
        Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
        Nayna Jain <nayna@...ux.ibm.com>, linuxppc-dev@...abs.org,
        linux-efi@...r.kernel.org, linux-integrity@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Jeremy Kerr <jk@...abs.org>,
        Matthew Garret <matthew.garret@...ula.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Claudio Carvalho <cclaudio@...ux.ibm.com>,
        George Wilson <gcwilson@...ux.ibm.com>,
        Elaine Palmer <erpalmer@...ibm.com>,
        Eric Ricther <erichte@...ux.ibm.com>,
        "Oliver O'Halloran" <oohall@...il.com>,
        Prakhar Srivastava <prsriva02@...il.com>
Subject: Re: [PATCH v9 2/8] powerpc/ima: add support to initialize ima
 policy rules

On Sat, 2019-10-26 at 19:52 -0400, Mimi Zohar wrote:
> On Fri, 2019-10-25 at 12:02 -0500, Nayna Jain wrote:
> > On 10/24/19 12:35 PM, Lakshmi Ramasubramanian wrote:
> > > On 10/23/2019 8:47 PM, Nayna Jain wrote:
> > >
> > >> +/*
> > >> + * The "secure_rules" are enabled only on "secureboot" enabled systems.
> > >> + * These rules verify the file signatures against known good values.
> > >> + * The "appraise_type=imasig|modsig" option allows the known good 
> > >> signature
> > >> + * to be stored as an xattr or as an appended signature.
> > >> + *
> > >> + * To avoid duplicate signature verification as much as possible, 
> > >> the IMA
> > >> + * policy rule for module appraisal is added only if 
> > >> CONFIG_MODULE_SIG_FORCE
> > >> + * is not enabled.
> > >> + */
> > >> +static const char *const secure_rules[] = {
> > >> +    "appraise func=KEXEC_KERNEL_CHECK appraise_type=imasig|modsig",
> > >> +#ifndef CONFIG_MODULE_SIG_FORCE
> > >> +    "appraise func=MODULE_CHECK appraise_type=imasig|modsig",
> > >> +#endif
> > >> +    NULL
> > >> +};
> > >
> > > Is there any way to not use conditional compilation in the above array 
> > > definition? Maybe define different functions to get "secure_rules" for 
> > > when CONFIG_MODULE_SIG_FORCE is defined and when it is not defined.
> > 
> > How will you decide which function to be called ?
> 
> You could call "is_module_sig_enforced()".

Calling is_module_sig_enforce() would prevent verifying the same
kernel module appended signature twice, when CONFIG_MODULE_SIG is
enabled, but not CONFIG_MODULE_SIG_FORCE.  This comes at the expense
of having to define additional policies.

Unlike for the kernel image, there is no coordination between lockdown
and IMA for kernel modules signature verification.  I suggest
deferring defining additional policies to when the lockdown/IMA
coordination is addressed.

Mimi