Re: [PATCH v3 3/3] arm64/ima: add ima_arch support

[Chester Lin]

On Fri, Oct 30, 2020 at 12:53:25PM +0100, Ard Biesheuvel wrote:
> On Fri, 30 Oct 2020 at 07:09, Chester Lin <clin@xxxxxxxx> wrote:
> >
> > Add arm64 IMA arch support. The code and arch policy is mainly inherited
> > from x86.
> >
> > Signed-off-by: Chester Lin <clin@xxxxxxxx>
> > ---
> >  arch/arm64/Kconfig           |  1 +
> >  arch/arm64/kernel/Makefile   |  2 ++
> >  arch/arm64/kernel/ima_arch.c | 43 ++++++++++++++++++++++++++++++++++++
> >  3 files changed, 46 insertions(+)
> >  create mode 100644 arch/arm64/kernel/ima_arch.c
> >
> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > index a42e8d13cc88..496a4a26afc6 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -201,6 +201,7 @@ config ARM64
> >         select SWIOTLB
> >         select SYSCTL_EXCEPTION_TRACE
> >         select THREAD_INFO_IN_TASK
> > +       imply IMA_SECURE_AND_OR_TRUSTED_BOOT if EFI
> >         help
> >           ARM 64-bit (AArch64) Linux support.
> >
> > diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
> > index bbaf0bc4ad60..0f6cbb50668c 100644
> > --- a/arch/arm64/kernel/Makefile
> > +++ b/arch/arm64/kernel/Makefile
> > @@ -69,3 +69,5 @@ extra-y                                       += $(head-y) vmlinux.lds
> >  ifeq ($(CONFIG_DEBUG_EFI),y)
> >  AFLAGS_head.o += -DVMLINUX_PATH="\"$(realpath $(objtree)/vmlinux)\""
> >  endif
> > +
> > +obj-$(CONFIG_IMA_SECURE_AND_OR_TRUSTED_BOOT)   += ima_arch.o
> > diff --git a/arch/arm64/kernel/ima_arch.c b/arch/arm64/kernel/ima_arch.c
> > new file mode 100644
> > index 000000000000..564236d77adc
> > --- /dev/null
> > +++ b/arch/arm64/kernel/ima_arch.c
> > @@ -0,0 +1,43 @@
> > +// SPDX-License-Identifier: GPL-2.0+
> > +/*
> > + * Copyright (C) 2018 IBM Corporation
> > + */
> > +#include <linux/efi.h>
> > +#include <linux/module.h>
> > +#include <linux/ima.h>
> > +
> > +bool arch_ima_get_secureboot(void)
> > +{
> > +       static bool sb_enabled;
> > +       static bool initialized;
> > +
> > +       if (!initialized & efi_enabled(EFI_BOOT)) {
> > +               sb_enabled = ima_get_efi_secureboot();
> > +               initialized = true;
> > +       }
> > +
> > +       return sb_enabled;
> > +}
> > +
> > +/* secure and trusted boot arch rules */
> > +static const char * const sb_arch_rules[] = {
> > +#if !IS_ENABLED(CONFIG_KEXEC_SIG)
> > +       "appraise func=KEXEC_KERNEL_CHECK appraise_type=imasig",
> > +#endif /* CONFIG_KEXEC_SIG */
> > +       "measure func=KEXEC_KERNEL_CHECK",
> > +#if !IS_ENABLED(CONFIG_MODULE_SIG)
> > +       "appraise func=MODULE_CHECK appraise_type=imasig",
> > +#endif
> > +       "measure func=MODULE_CHECK",
> > +       NULL
> > +};
> > +
> > +const char * const *arch_get_ima_policy(void)
> > +{
> > +       if (IS_ENABLED(CONFIG_IMA_ARCH_POLICY) && arch_ima_get_secureboot()) {
> > +               if (IS_ENABLED(CONFIG_MODULE_SIG))
> > +                       set_module_sig_enforced();
> > +               return sb_arch_rules;
> > +       }
> > +       return NULL;
> > +}
> > --
> > 2.28.0
> >
> 
> Can we move all this stuff into security/integrity/ima/ima_efi.c instead?
>
Actually I hesitated to move all this stuff into ima_efi.c when coding v3
because I haven't figured out a clear picture to achieve it. Since each
architecture could still have different details to trigger secure boot detection
and define their arch-specific rules [e.g. Having boot_params in x86_64 creates
more conditions that need to be determined before calling get_sb_mode()], moving
all this stuff seems to decrease the flexibility. Besides, it might also affect
the consistency of ima_arch as well, for example, ppc and s390 still use these
function prototypes defined in ima.h. Since these functions are already referred
by non-EFI architectures, why don't we still reuse these prototypes? For example,
we could remain a smaller arch_ima_get_secureboot() and the arch-specific rules
but move the major part of arch_get_ima_policy() into ima_efi.c. For example,
we could implement an efi_ima_policy() for arch_get_ima_policy() to call so that
the arch_get_ima_policy() doesn't have to know some details such as checking
conditions or calling set_module_sig_enforced().

Please feel free to let me know if any suggestions.