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

[Nayna]

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.

Thanks & Regards,
ÂÂ - Nayna