Re: [RFC] EVM: Add support for portable signature format

[Matthew Garrett <mjg59@google.com>]

On Thu, Oct 26, 2017 at 2:03 AM, Mikhail Kurinnoi
<viewizard@viewizard.com> wrote:
> ? Thu, 26 Oct 2017 01:31:44 -0700
> Matthew Garrett <mjg59@google.com> ?????:
>
>> @@ -317,7 +319,7 @@ void ima_update_xattr(struct integrity_iint_cache
>> *iint, struct file *file) int rc = 0;
>>
>>       /* do not collect and update hash for digital signatures */
>> -     if (iint->flags & IMA_DIGSIG)
>> +     if (iint->flags & IMA_DIGSIG || iint->flags &
>> EVM_IMMUTABLE_DIGSIG) return;
>>
>
> Isn't this mean, we already changed files data, and we just don't allow
> IMA xattr update? This file will not pass integrity verification
> next time.

That's fine - policy may not care. It's easier to sign all files and
then leave enforcement up to the local policy than it is to determine
in advance which files will need protection.

> I thought, the idea was prevent data changes, and in this way prevent
> IMA xattr update.

No, the goal is to be able to detect when files have been modified and
(optionally) restrict access as a result. Otherwise the packaging
system has to be able to identify all files that may be legitimately
modified, which is something that may differ depending on the client.
It's much easier to permit the data modification and have the local
policy block reading or execution if it's actually a sensitive file.