Re: [PATCH v6 3/9] powerpc: add support to initialize ima policy rules

[Nayna]

On 10/02/2019 05:49 PM, Mimi Zohar wrote:
> On Tue, 2019-10-01 at 12:07 -0400, Nayna wrote:
> > On 09/30/2019 09:04 PM, Thiago Jung Bauermann wrote:
> > > Hello,
> > Hi,
> >
> > > > diff --git a/arch/powerpc/kernel/ima_arch.c b/arch/powerpc/kernel/ima_arch.c
> > > > new file mode 100644
> > > > index 000000000000..39401b67f19e
> > > > --- /dev/null
> > > > +++ b/arch/powerpc/kernel/ima_arch.c
> > > > @@ -0,0 +1,33 @@
> > > > +// SPDX-License-Identifier: GPL-2.0
> > > > +/*
> > > > + * Copyright (C) 2019 IBM Corporation
> > > > + * Author: Nayna Jain
> > > > + */
> > > > +
> > > > +#include <linux/ima.h>
> > > > +#include <asm/secure_boot.h>
> > > > +
> > > > +bool arch_ima_get_secureboot(void)
> > > > +{
> > > > +	return is_powerpc_os_secureboot_enabled();
> > > > +}
> > > > +
> > > > +/* Defines IMA appraise rules for secureboot */
> > > > +static const char *const arch_rules[] = {
> > > > +	"appraise func=KEXEC_KERNEL_CHECK appraise_type=imasig|modsig",
> > > > +#if !IS_ENABLED(CONFIG_MODULE_SIG)
> > > > +	"appraise func=MODULE_CHECK appraise_type=imasig|modsig",
> > > > +#endif
> > > > +	NULL
> > > > +};
> > > > +
> > > > +/*
> > > > + * Returns the relevant IMA arch policies based on the system secureboot state.
> > > > + */
> > > > +const char *const *arch_get_ima_policy(void)
> > > > +{
> > > > +	if (is_powerpc_os_secureboot_enabled())
> > > > +		return arch_rules;
> > > > +
> > > > +	return NULL;
> > > > +}
> > > If CONFIG_MODULE_SIG is enabled but module signatures aren't enforced,
> > > then IMA won't enforce module signature either. x86's
> > > arch_get_ima_policy() calls set_module_sig_enforced(). Doesn't the
> > > powerpc version need to do that as well?
> > >
> > > On the flip side, if module signatures are enforced by the module
> > > subsystem then IMA will verify the signature a second time since there's
> > > no sharing of signature verification results between the module
> > > subsystem and IMA (this was observed by Mimi).
> > >
> > > IMHO this is a minor issue, since module loading isn't a hot path and
> > > the duplicate work shouldn't impact anything. But it could be avoided by
> > > having a NULL entry in arch_rules, which arch_get_ima_policy() would
> > > dynamically update with the "appraise func=MODULE_CHECK" rule if
> > > is_module_sig_enforced() is true.
> > Thanks Thiago for reviewing. I am wondering that this will give two
> > meanings for NULL. Can we do something like below, there are possibly
> > two options ?
> >
> > 1. Set IMA_APPRAISED in the iint->flags if is_module_sig_enforced().
> >
> > OR
> >
> > 2. Let ima_get_action() check for is_module_sig_enforced() when policy
> > is appraise and func is MODULE_CHECK.
> I'm a bit hesitant about mixing the module subsystem signature
> verification method with the IMA measure "template=ima-modsig" rules.
>  ÂDoes it actually work?
>
> We can at least limit verifying the same appended signature twice to
> when "module.sig_enforce" is specified on the boot command line, by
> changing "!IS_ENABLED(CONFIG_MODULE_SIG)" to test
> "CONFIG_MODULE_SIG_FORCE".

Yes this seems to be a better idea. I have implemented this in the v7 
version of the ima_arch version.

Thanks & Regards,
ÂÂÂÂ - Nayna