by Nayna Jain

[permalink] [raw]

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

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.
> Does 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,
- Nayna