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

[Eric DeVolder <eric.devolder@oracle.com>]

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
>>