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

Stefan Berger
2025-01-28T17:30:41Z

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.
>
>