Messages in this thread | | | Date | Fri, 11 Feb 2022 16:31:00 +0100 | Subject | Re: [PATCH v5 2/6] powerpc/kexec_file: Add KEXEC_SIG support. | From | Paul Menzel <> |
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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
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