Re: [dm-crypt] how to get keyslog PBKDF settings via libcryptsetup

Jonas Meurer <[email protected]> Wed, 29 Jan 2020 15:14:24 +0100
Newsgroups	de.saout.dm-crypt
Message-ID	<[email protected]>
Hey Ondrej,

Ondrej Kozina:
> On 1/28/20 3:51 PM, Jonas Meurer wrote:
>> Hello dm-crypt folks,
>>
>> I want to retrieve pbkdf params for active keyslots of LUKS devices.
>> First I looked into `crypt_keyslot_get_pbkdf()`, but contrary to what
>> the code suggests[1], it doesn't return values for LUKS1 devices.
> 
> No, it should definitely work. What version of library have you tested
> it with?

Thanks for your comments and confirming that I'm on the right track :)

I'm using the latest cryptsetup 2.2.2 version (from the Debian packages).

> So, are you sure *cd contains proper LUKS1 context?

At least I hope so. `crypt_get_type(cd)` returns 'LUKS1' and 'LUKS2'
respectively.

> Probably, you've just switched the condition on
> crypt_keyslot_get_pbkdf(). It returns 0 on success and negative value
> otherwise.

That's how I first understood it that way as well. But after some
testing I got the impression that my crypt_pbkdf_type struct contains
values if the return code is *not* 0. Anyway, I checked the return codes
of `crypt_keyslot_get_pbkdf(cd, ki, &pbkdf_ki)` now for LUKS1 and LUKS2
devices and interestingly, I always get return code '0' for LUKS1
devices and '-2' for LUKS2 devices:

Device cont1_crypt (type LUKS2)
Active keyslot 0: 2
  return code: -2
Active keyslot 1: 2
  return code: -2

Device cont3_crypt (type LUKS1)
Active keyslot 0: 3
cryptsetup-suspend: couldn't get PBKDF for keyslot 0 of device
cont3_crypt: Success
  return code: 0

Also, the crypt_pbkdf_type struct has values '0' for all integers for
LUKS1 and seems to have random values for LUKS2 (probably pointers to
random memory pages?).

Simplified code:

struct crypt_device *cd = NULL;
crypt_init_by_name(&cd, devices[i]);
printf("Device %s (type %s)\n", devices[i], crypt_get_type(cd));
int ks_max = crypt_keyslot_max(crypt_get_type(cd));
for (int j = 0; j < ks_max; j++) {
    crypt_keyslot_info ki = crypt_keyslot_status(cd, j);
    if (ki != CRYPT_SLOT_ACTIVE && ki != CRYPT_SLOT_ACTIVE_LAST)
        continue;
    printf("Active keyslot %d: %d\n", j, ki);
    struct crypt_pbkdf_type pbkdf_ki;
    int res = crypt_keyslot_get_pbkdf(cd, ki, &pbkdf_ki);
    printf("  return code: %d\n", res);
}
crypt_free(cd);


>> Also, looking at the actual return valudes, it seems to return the
>> calculated values for a new keyslot, not the ones for the active
>> keyslot, right?
> 
> Again, this would be either bug in libcryptsetup or some mistake in your
> application code. Looking at libcryptsetup code it extracts values
> directly from metadata for both LUKS1 and LUKS2.

Yep, apparently either I'm doing something wrong or there's a bug in
libcryptsetup. Unfortunately I'm not sure which one is true ;)

> Do you get same values when you try to modify your code a bit?
> 
> Could you replace crypt_init_by_name() with
> 
> crypt_init(/path/to/luks/metadata/device)
> and subsequent
> crypt_load()
> 
> Do you get same (wrong) values?

Jep, I now tried that with exactly the same results.

Cheers
 jonas
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