Date:   Fri, 11 Feb 2022 16:31:00 +0100
From:   Paul Menzel <pmenzel@...gen.mpg.de>
To:     Michal Suchánek <msuchanek@...e.de>
Cc:     keyrings@...r.kernel.org, linux-crypto@...r.kernel.org,
        linux-integrity@...r.kernel.org, kexec@...ts.infradead.org,
        Philipp Rudo <prudo@...hat.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Nayna <nayna@...ux.vnet.ibm.com>, Rob Herring <robh@...nel.org>,
        linux-s390@...r.kernel.org, Vasily Gorbik <gor@...ux.ibm.com>,
        Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Jessica Yu <jeyu@...nel.org>, linux-kernel@...r.kernel.org,
        David Howells <dhowells@...hat.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Paul Mackerras <paulus@...ba.org>,
        Hari Bathini <hbathini@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org,
        Frank van der Linden <fllinden@...zon.com>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        Daniel Axtens <dja@...ens.net>, buendgen@...ibm.com,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Baoquan He <bhe@...hat.com>,
        linux-security-module@...r.kernel.org
Subject: Re: [PATCH v5 2/6] powerpc/kexec_file: Add KEXEC_SIG support.

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@...e.de>
>>> ---
>>> v3: - Philipp Rudo <prudo@...hat.com>: Update the comit message with
>>>         explanation why the s390 code is usable on powerpc.
>>>       - Include correct header for mod_check_sig
>>>       - Nayna <nayna@...ux.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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