Re: [PATCH v13 8/9] ima: make the kexec extra memory configurable

[steven chen <chenste@linux.microsoft.com>]

On 4/29/2025 12:06 PM, Stefan Berger wrote:
>
>
> On 4/21/25 6:25 PM, steven chen wrote:
>> From: Steven Chen <chenste@linux.microsoft.com>
>>
>> The extra memory allocated for carrying the IMA measurement list across
>> kexec is hard-coded as half a PAGE.  Make it configurable.
>>
>> Define a Kconfig option, IMA_KEXEC_EXTRA_MEMORY_KB, to configure the
>> extra memory (in kb) to be allocated for IMA measurements added during
>> kexec soft reboot.  Ensure the default value of the option is set such
>> that extra half a page of memory for additional measurements is 
>> allocated
>> for the additional measurements.
>>
>> Update ima_add_kexec_buffer() function to allocate memory based on the
>> Kconfig option value, rather than the currently hard-coded one.
>>
>> Suggested-by: Stefan Berger <stefanb@linux.ibm.com>
>> Co-developed-by: Tushar Sugandhi <tusharsu@linux.microsoft.com>
>> Signed-off-by: Tushar Sugandhi <tusharsu@linux.microsoft.com>
>> Signed-off-by: Steven Chen <chenste@linux.microsoft.com>
>> Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
>> Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
>> Acked-by: Baoquan He <bhe@redhat.com>
>> ---
>>   security/integrity/ima/Kconfig     | 11 +++++++++++
>>   security/integrity/ima/ima_kexec.c | 16 +++++++++++-----
>>   2 files changed, 22 insertions(+), 5 deletions(-)
>>
>> diff --git a/security/integrity/ima/Kconfig 
>> b/security/integrity/ima/Kconfig
>> index 475c32615006..976e75f9b9ba 100644
>> --- a/security/integrity/ima/Kconfig
>> +++ b/security/integrity/ima/Kconfig
>> @@ -321,4 +321,15 @@ config IMA_DISABLE_HTABLE
>>       help
>>          This option disables htable to allow measurement of 
>> duplicate records.
>>   +config IMA_KEXEC_EXTRA_MEMORY_KB
>> +    int "Extra memory for IMA measurements added during kexec soft 
>> reboot"
>> +    range 0 40
>> +    depends on IMA_KEXEC
>> +    default 0
>> +    help
>> +      IMA_KEXEC_EXTRA_MEMORY_KB determines the extra memory to be
>> +      allocated (in kb) for IMA measurements added during kexec soft 
>> reboot.
>> +      If set to the default value of 0, an extra half page of memory 
>> for those
>> +      additional measurements will be allocated.
>
> If you have an IMA policy taking quite a few measurements and you are 
> fast after reboot to log in to initiate the 'kexec load' (While system 
> is still starting up), the system may end up with loss of measurements 
> very easily if the default is 0 and pages are small. -> Set the 
> default to the max? Also, would we expect distros to all go through 
> the new config option and choose 40 or will they likely leave it at 0?
>
Hi Stefan,

Could you please check the comments of version V11 on this default value?

https://lore.kernel.org/all/20250402124725.5601-1-chenste@linux.microsoft.com/

Thanks,

Steven

>> +
>>   endif
>> diff --git a/security/integrity/ima/ima_kexec.c 
>> b/security/integrity/ima/ima_kexec.c
>> index ed867734ee70..d1c9d369ba08 100644
>> --- a/security/integrity/ima/ima_kexec.c
>> +++ b/security/integrity/ima/ima_kexec.c
>> @@ -118,6 +118,7 @@ void ima_add_kexec_buffer(struct kimage *image)
>>                     .buf_min = 0, .buf_max = ULONG_MAX,
>>                     .top_down = true };
>>       unsigned long binary_runtime_size;
>> +    unsigned long extra_memory;
>>         /* use more understandable variable names than defined in 
>> kbuf */
>>       size_t kexec_buffer_size = 0;
>> @@ -125,15 +126,20 @@ void ima_add_kexec_buffer(struct kimage *image)
>>       int ret;
>>         /*
>> -     * Reserve an extra half page of memory for additional measurements
>> -     * added during the kexec load.
>> +     * Reserve extra memory for measurements added during kexec.
>>        */
>> -    binary_runtime_size = ima_get_binary_runtime_size();
>> +    if (CONFIG_IMA_KEXEC_EXTRA_MEMORY_KB <= 0)
>> +        extra_memory = PAGE_SIZE / 2;
>> +    else
>> +        extra_memory = CONFIG_IMA_KEXEC_EXTRA_MEMORY_KB * 1024;
>> +
>> +    binary_runtime_size = ima_get_binary_runtime_size() + extra_memory;
>> +
>>       if (binary_runtime_size >= ULONG_MAX - PAGE_SIZE)
>>           kexec_segment_size = ULONG_MAX;
>>       else
>> -        kexec_segment_size = ALIGN(ima_get_binary_runtime_size() +
>> -                       PAGE_SIZE / 2, PAGE_SIZE);
>> +        kexec_segment_size = ALIGN(binary_runtime_size, PAGE_SIZE);
>> +
>>       if ((kexec_segment_size == ULONG_MAX) ||
>>           ((kexec_segment_size >> PAGE_SHIFT) > totalram_pages() / 2)) {
>>           pr_err("Binary measurement list too large.\n");