Re: [PATCH v7 2/8] powerpc: add support to initialize ima policy rules

[Michael Ellerman <mpe@ellerman.id.au>]

Nayna Jain <nayna@linux.ibm.com> writes:
> PowerNV systems uses kernel based bootloader, thus its secure boot
> implementation uses kernel IMA security subsystem to verify the kernel
> before kexec. Since the verification policy might differ based on the
> secure boot mode of the system, the policies are defined at runtime.
>
> This patch implements the arch-specific support to define the IMA policy
> rules based on the runtime secure boot mode of the system.
>
> This patch provides arch-specific IMA policies if PPC_SECURE_BOOT
> config is enabled.
...
> diff --git a/arch/powerpc/kernel/ima_arch.c b/arch/powerpc/kernel/ima_arch.c
> new file mode 100644
> index 000000000000..c22d82965eb4
> --- /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_FORCE)
> +	"appraise func=MODULE_CHECK appraise_type=imasig|modsig",
> +#endif

This confuses me.

If I spell it out we get:

#if IS_ENABLED(CONFIG_MODULE_SIG_FORCE)
	// nothing
#else
	"appraise func=MODULE_CHECK appraise_type=imasig|modsig",
#endif

Which is just:

#ifdef CONFIG_MODULE_SIG_FORCE
	// nothing
#else
	"appraise func=MODULE_CHECK appraise_type=imasig|modsig",
#endif

But CONFIG_MODULE_SIG_FORCE enabled says that we *do* require modules to
have a valid signature. Isn't that the inverse of what the rules say?

Presumably I'm misunderstanding something :)

cheers