Re: [PATCH v2 12/13] s390/kexec: refactor for kernel/Kconfig.kexec

Eric DeVolder
2023-06-21T17:11:55Z

On 6/21/23 00:00, Alexander Gordeev wrote:
> On Mon, Jun 19, 2023 at 10:58:00AM -0400, Eric DeVolder wrote:
>
> Hi Eric,
>
>> The kexec and crash kernel options are provided in the common
>> kernel/Kconfig.kexec. Utilize the common options and provide
>> the ARCH_SUPPORTS_ and ARCH_SELECTS_ entries to recreate the
>> equivalent set of KEXEC and CRASH options.
>>
>> NOTE: The original Kconfig has a KEXEC_SIG which depends on
>> MODULE_SIG_FORMAT. However, attempts to keep the MODULE_SIG_FORMAT
>> dependency (using the strategy outlined in this series, and other
>> techniques) results in 'error: recursive dependency detected'
>> on CRYPTO. This occurs due to any path through KEXEC_SIG
>> attempting to select CRYPTO is ultimately dependent upon CRYPTO:
>>
>> CRYPTO
>> <- ARCH_SUPPORTS_KEXEC_FILE
>> <- KEXEC_FILE
>> <- KEXEC_SIG
>>
>> Therefore, the solution is to drop the MODULE_SIG_FORMAT dependency
>> for KEXEC_SIG. In practice, however, MODULE_SIG_FORMAT is still
>> configured-in as the use of KEXEC_SIG is in step with the use of
>> SYSTEM_DATA_VERIFICATION, which does select MODULE_SIG_FORMAT.
>
> No, it is actually the other way around.
> Could you please provide the correct explanation?
>
> AFAICT the MODULE_SIG_FORMAT dependency was introduced with commit
> c8424e776b09 ("MODSIGN: Export module signature definitions") and
> in fact was not necessary, since s390 did/does not use mod_check_sig()
> anyway. So the SYSTEM_DATA_VERIFICATION could have left intact.

Thomas, would the correct explanation be simply indicating that MODULE_SIG_FORMAT isn't needed as it
is not used by s390 (crediting your summary above)?

>
> However, the original SYSTEM_DATA_VERIFICATION seems sane and I do
> not understand why other architectures do not have it also? May be
> Mimi Zohar (putting on CC) could explain that?
>
> It looks like such dependency actually exists in implicit form
> (which you picked from x86):
>
> In addition to this option, you need to enable signature
> verification for the corresponding kernel image type being
> loaded in order for this to work.
>
> Does it mean that if an architecture did not enable the signature
> verification type explicitly the linker could fail - both before
> and after you series?

As a quick test I checked x86 and it compiles/links ok if KEXEC_SIG and KEXEC_SIG_FORCE are
configured-in, but KEXEC_BZIMAGE_VERIFY_SIG (used for x86 sig verify) is not. The reason being that
the kexec_image_verify_sig() function checks if the fops.verify_sig is non-NULL before invoking the
verification. If it is NULL, the sig check fails. This would appear to be valid outcome for other
archs as well.

At any rate, I think attempting to determine if other archs need SYSTEM_DATA_VERIFICATION is out of
the scope of this series; I'm targeting just the refactor to be equivalent to what is what prior.

Thanks for looking at this!
eric
>
> Thanks!
>
>> Not ideal, but results in equivalent .config files for s390.
>>
>> Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
>> ---
>> arch/s390/Kconfig | 65 ++++++++++++++---------------------------------
>> 1 file changed, 19 insertions(+), 46 deletions(-)
>>
>> diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
>> index 6dab9c1be508..58dc124433ca 100644
>> --- a/arch/s390/Kconfig
>> +++ b/arch/s390/Kconfig
>> @@ -243,6 +243,25 @@ config PGTABLE_LEVELS
>>
>> source "kernel/livepatch/Kconfig"
>>
>> +config ARCH_DEFAULT_KEXEC
>> + def_bool y
>> +
>> +config ARCH_SUPPORTS_KEXEC
>> + def_bool y
>> +
>> +config ARCH_SUPPORTS_KEXEC_FILE
>> + def_bool CRYPTO && CRYPTO_SHA256 && CRYPTO_SHA256_S390
>> +
>> +config ARCH_HAS_KEXEC_PURGATORY
>> + def_bool KEXEC_FILE
>> +
>> +config ARCH_SUPPORTS_CRASH_DUMP
>> + def_bool y
>> + help
>> + Refer to <file:Documentation/s390/zfcpdump.rst> for more details on this.
>> + This option also enables s390 zfcpdump.
>> + See also <file:Documentation/s390/zfcpdump.rst>
>> +
>> menu "Processor type and features"
>>
>> config HAVE_MARCH_Z10_FEATURES
>> @@ -481,36 +500,6 @@ config SCHED_TOPOLOGY
>>
>> source "kernel/Kconfig.hz"
>>
>> -config KEXEC
>> - def_bool y
>> - select KEXEC_CORE
>> -
>> -config KEXEC_FILE
>> - bool "kexec file based system call"
>> - select KEXEC_CORE
>> - depends on CRYPTO
>> - depends on CRYPTO_SHA256
>> - depends on CRYPTO_SHA256_S390
>> - help
>> - Enable the kexec file based system call. In contrast to the normal
>> - kexec system call this system call takes file descriptors for the
>> - kernel and initramfs as arguments.
>> -
>> -config ARCH_HAS_KEXEC_PURGATORY
>> - def_bool y
>> - depends on 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.
>> -
>> config KERNEL_NOBP
>> def_bool n
>> prompt "Enable modified branch prediction for the kernel by default"
>> @@ -732,22 +721,6 @@ config VFIO_AP
>>
>> endmenu
>>
>> -menu "Dump support"
>> -
>> -config CRASH_DUMP
>> - bool "kernel crash dumps"
>> - select KEXEC
>> - help
>> - Generate crash dump after being started by kexec.
>> - Crash dump kernels are loaded in the main kernel with kexec-tools
>> - into a specially reserved region and then later executed after
>> - a crash by kdump/kexec.
>> - Refer to <file:Documentation/s390/zfcpdump.rst> for more details on this.
>> - This option also enables s390 zfcpdump.
>> - See also <file:Documentation/s390/zfcpdump.rst>
>> -
>> -endmenu
>> -
>> config CCW
>> def_bool y
>>
>> --
>> 2.31.1
>>