Re: [PATCH v5 2/6] powerpc/kexec_file: Add KEXEC_SIG support.

Paul Menzel
2022-02-11T15:31:09Z

Dear Michal,

Am 09.02.22 um 13:01 schrieb Michal Suchánek:

> On Wed, Feb 09, 2022 at 07:44:15AM +0100, Paul Menzel wrote:

>> Am 11.01.22 um 12:37 schrieb Michal Suchanek:

[…]

>> How can this be tested?
>
> Apparently KEXEC_SIG_FORCE is x86 only although the use of the option is
> arch neutral:
>
> arch/x86/Kconfig:config KEXEC_SIG_FORCE
> kernel/kexec_file.c: if (IS_ENABLED(CONFIG_KEXEC_SIG_FORCE))
> {
>
> Maybe it should be moved?

Sounds good.

> I used a patched kernel that enables lockdown in secure boot, and then
> verified that signed kernel can be loaded by kexec and unsigned not,
> with KEXEC_SIG enabled and IMA_KEXEC disabled.
>
> The lockdown support can be enabled on any platform, and although I
> can't find it documented anywhere there appears to be code in kexec_file
> to take it into account:
> kernel/kexec.c: result = security_locked_down(LOCKDOWN_KEXEC);
> kernel/kexec_file.c: security_locked_down(LOCKDOWN_KEXEC))
> kernel/module.c: return security_locked_down(LOCKDOWN_MODULE_SIGNATURE);
> kernel/params.c: security_locked_down(LOCKDOWN_MODULE_PARAMETERS))
> and lockdown can be enabled with a buildtime option, a kernel parameter, or a
> debugfs file.
>
> Still for testing lifting KEXEC_SIG_FORCE to some arch-neutral Kconfig file is
> probably the simplest option.
>
> kexec -s option should be used to select kexec_file rather than the old
> style kexec which would either fail always or succeed always regardelss
> of signature.

Thank you.

>>> Signed-off-by: Michal Suchanek <msuchanek@suse.de>
>>> ---
>>> v3: - Philipp Rudo <prudo@redhat.com>: Update the comit message with
>>> explanation why the s390 code is usable on powerpc.
>>> - Include correct header for mod_check_sig
>>> - Nayna <nayna@linux.vnet.ibm.com>: Mention additional IMA features
>>> in kconfig text
>>> ---
>>> arch/powerpc/Kconfig | 16 ++++++++++++++++
>>> arch/powerpc/kexec/elf_64.c | 36 ++++++++++++++++++++++++++++++++++++
>>> 2 files changed, 52 insertions(+)
>>>
>>> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
>>> index dea74d7717c0..1cde9b6c5987 100644
>>> --- a/arch/powerpc/Kconfig
>>> +++ b/arch/powerpc/Kconfig
>>> @@ -560,6 +560,22 @@ config KEXEC_FILE
>>> config ARCH_HAS_KEXEC_PURGATORY
>>> def_bool KEXEC_FILE
>>> +config KEXEC_SIG
>>> + bool "Verify kernel signature during kexec_file_load() syscall"
>>> + depends on KEXEC_FILE && MODULE_SIG_FORMAT
>>> + help
>>> + This option makes kernel signature verification mandatory for
>>> + the kexec_file_load() syscall.
>>> +
>>> + In addition to that option, you need to enable signature
>>> + verification for the corresponding kernel image type being
>>> + loaded in order for this to work.
>>> +
>>> + Note: on powerpc IMA_ARCH_POLICY also implements kexec'ed kernel
>>> + verification. In addition IMA adds kernel hashes to the measurement
>>> + list, extends IMA PCR in the TPM, and implements kernel image
>>> + blacklist by hash.
>>
>> So, what is the takeaway for the user? IMA_ARCH_POLICY is preferred? What is
>> the disadvantage, and two implementations(?) needed then? More overhead?
>
> IMA_KEXEC does more than KEXEC_SIG. The overhead is probably not big
> unless you are trying to really minimize the kernel code size.
>
> Arguably the simpler implementation has less potential for bugs, too.
> Both in code and in user configuration required to enable the feature.
>
> Interestingly IMA_ARCH_POLICY depends on KEXEC_SIG rather than
> IMA_KEXEC. Just mind-boggling.

I have not looked into that.

> The main problem with IMA_KEXEC from my point of view is it is not portable.
> To record the measurements TPM support is requireed which is not available on
> all platforms. It does not support PE so it cannot be used on platforms
> that use PE kernel signature format.

Could you add that to the comment please?

>>> +
>>> config RELOCATABLE
>>> bool "Build a relocatable kernel"
>>> depends on PPC64 || (FLATMEM && (44x || FSL_BOOKE))
>>> diff --git a/arch/powerpc/kexec/elf_64.c b/arch/powerpc/kexec/elf_64.c
>>> index eeb258002d1e..98d1cb5135b4 100644
>>> --- a/arch/powerpc/kexec/elf_64.c
>>> +++ b/arch/powerpc/kexec/elf_64.c
>>> @@ -23,6 +23,7 @@
>>> #include <linux/of_fdt.h>
>>> #include <linux/slab.h>
>>> #include <linux/types.h>
>>> +#include <linux/module_signature.h>
>>> static void *elf64_load(struct kimage *image, char *kernel_buf,
>>> unsigned long kernel_len, char *initrd,
>>> @@ -151,7 +152,42 @@ static void *elf64_load(struct kimage *image, char *kernel_buf,
>>> return ret ? ERR_PTR(ret) : NULL;
>>> }
>>> +#ifdef CONFIG_KEXEC_SIG
>>> +int elf64_verify_sig(const char *kernel, unsigned long kernel_len)
>>> +{
>>> + const unsigned long marker_len = sizeof(MODULE_SIG_STRING) - 1;
>>> + struct module_signature *ms;
>>> + unsigned long sig_len;
>>
>> Use size_t to match the signature of `verify_pkcs7_signature()`?
>
> Nope. struct module_signature uses unsigned long, and this needs to be
> matched to avoid type errors on 32bit.

I meant for `sig_len`.

> Technically using size_t for in-memory buffers is misguided because
> AFAICT no memory buffer can be bigger than ULONG_MAX, and size_t is
> non-native type on 32bit.
>
> Sure, the situation with ssize_t/int is different but that's not what we
> are dealing with here.
True. In my experience it prevents compiler warnings when building for
32 bit or 64 bit. Anyway, not that important.

Kind regards,

Paul