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

Nayna
2019-10-01T16:07:44Z

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,
- Nayna