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

[Mimi Zohar]

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".

Mimi