Re: [PATCH 7/7] ima: measure kexec load and exec events as critical data

steven chen
2025-01-28T18:35:06Z

On 1/28/2025 9:29 AM, Stefan Berger wrote:
>
>
> On 1/28/25 10:28 AM, Stefan Berger wrote:
>>
>>
>> On 1/24/25 5:55 PM, steven chen wrote:
>>> The amount of memory allocated at kexec load, even with the extra
>>> memory
>>> allocated, might not be large enough for the entire measurement
>>> list.  The
>>> indeterminate interval between kexec 'load' and 'execute' could
>>> exacerbate
>>> this problem.
>>>
>>> Define two new IMA events, 'kexec_load' and 'kexec_execute', to be
>>> measured as critical data at kexec 'load' and 'execute' respectively.
>>> Report the allocated kexec segment size, IMA binary log size and the
>>> runtime measurements count as part of those events.
>>>
>>> These events, and the values reported through them, serve as markers in
>>> the IMA log to verify the IMA events are captured during kexec soft
>>> reboot.  The presence of a 'kexec_load' event in between the last two
>>> 'boot_aggregate' events in the IMA log implies this is a kexec soft
>>> reboot, and not a cold-boot. And the absence of 'kexec_execute' event
>>> after kexec soft reboot implies missing events in that window which
>>> results in inconsistency with TPM PCR quotes, necessitating a cold boot
>>> for a successful remote attestation.
>>>
>>> Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
>>> Author: Tushar Sugandhi <tusharsu@linux.microsoft.com>
>>> Signed-off-by: Tushar Sugandhi <tusharsu@linux.microsoft.com>
>>> Signed-off-by: steven chen <chenste@linux.microsoft.com>
>>> ---
>>>   security/integrity/ima/ima_kexec.c | 23 +++++++++++++++++++++++
>>>   1 file changed, 23 insertions(+)
>>>
>>> diff --git a/security/integrity/ima/ima_kexec.c
>>> b/security/integrity/ ima/ima_kexec.c
>>> index c9c916f69ca7..d416ca0382cb 100644
>>> --- a/security/integrity/ima/ima_kexec.c
>>> +++ b/security/integrity/ima/ima_kexec.c
>>> @@ -17,6 +17,8 @@
>>>   #include "ima.h"
>>>   #ifdef CONFIG_IMA_KEXEC
>>> +#define IMA_KEXEC_EVENT_LEN 256
>>> +
>>>   static struct seq_file ima_kexec_file;
>>>   static void *ima_kexec_buffer;
>>>   static size_t kexec_segment_size;
>>> @@ -36,6 +38,24 @@ static void ima_free_kexec_file_buf(struct
>>> seq_file *sf)
>>>       ima_reset_kexec_file(sf);
>>>   }
>>> +static void ima_measure_kexec_event(const char *event_name)
>>> +{
>>> +    char ima_kexec_event[IMA_KEXEC_EVENT_LEN];
>>> +    size_t buf_size = 0;
>>> +    long len;
>>> +
>>> +    buf_size = ima_get_binary_runtime_size();
>>> +    len = atomic_long_read(&ima_htable.len);
>>> +
>>> +    scnprintf(ima_kexec_event, IMA_KEXEC_EVENT_LEN,
>>> + "kexec_segment_size=%lu;ima_binary_runtime_size=%lu;"
>>> +            "ima_runtime_measurements_count=%ld;",
>>> +                kexec_segment_size, buf_size, len);
>>> +
>>> +    ima_measure_critical_data("ima_kexec", event_name,
>>> ima_kexec_event,
>>> +                    strlen(ima_kexec_event), false, NULL, 0);
>>> +}
>>> +
>>>   static int ima_alloc_kexec_file_buf(size_t segment_size)
>>>   {
>>>       /*
>>> @@ -60,6 +80,7 @@ static int ima_alloc_kexec_file_buf(size_t
>>> segment_size)
>>>   out:
>>>       ima_kexec_file.read_pos = 0;
>>>       ima_kexec_file.count = sizeof(struct ima_kexec_hdr); /*
>>> reserved space */
>>> +    ima_measure_kexec_event("kexec_load");
>>>       return 0;
>>>   }
>>> @@ -206,6 +227,8 @@ static int ima_update_kexec_buffer(struct
>>> notifier_block *self,
>>>       if (ret)
>>>           pr_err("Dump measurements failed. Error:%d\n", ret);
>>> +    else
>>> +        ima_measure_kexec_event("kexec_execute");
>
> The problem is that this here ^^^^ currently comes after this block:
>
>         ret = ima_dump_measurement_list(&buf_size, &buf,
>                                         kexec_segment_size);
>
>         if (ret)
>                 pr_err("Dump measurements failed. Error:%d\n", ret);
>         else
>                 ima_measure_kexec_event("kexec_execute");  <--- after
> the dump
>
> It has to be done before so that whatever it adds to the measurement
> list gets dumped as well. So this works for my testing:
>
> +       ima_measure_kexec_event("kexec_execute");
> +
>         ret = ima_dump_measurement_list(&buf_size, &buf,
>                                         kexec_segment_size);
>
>         if (ret)
>                 pr_err("Dump measurements failed. Error:%d\n", ret);
>
>
>>>       if (buf_size != 0)
>>>           memcpy(ima_kexec_buffer, buf, buf_size);
>>
>>
>> I have been doing kexec's (on ppc64 KVM) applying one patch after
>> another in this series and then testing with this command:
>>
>> evmctl ima_measurement --ignore-violations /sys/kernel/security/ima/
>> binary_runtime_measurements
>>
>> Unfortunately it breaks at this patch. I am not sure what it is due to.
>>
>>
Thanks Stefan. Will update it in next release