Re: [PATCH v2] fscrypt: support trusted keys

[Ahmad Fatoum]

On 09.08.21 12:00, Ahmad Fatoum wrote:
> Hello Jarkko,
> 
> On 09.08.21 11:44, Jarkko Sakkinen wrote:
>> On Fri, Aug 06, 2021 at 05:09:28PM +0200, Ahmad Fatoum wrote:
>>> Kernel trusted keys don't require userspace knowledge of the raw key
>>> material and instead export a sealed blob, which can be persisted to
>>> unencrypted storage. Userspace can then load this blob into the kernel,
>>> where it's unsealed and from there on usable for kernel crypto.
>>>
>>> This is incompatible with fscrypt, where userspace is supposed to supply
>>> the raw key material. For TPMs, a work around is to do key unsealing in
>>> userspace, but this may not be feasible for other trusted key backends.
>>>
>>> Make it possible to benefit from both fscrypt and trusted key sealing
>>> by extending fscrypt_add_key_arg::key_id to hold either the ID of a
>>> fscrypt-provisioning or a trusted key.
>>>
>>> A non fscrypt-provisioning key_id was so far prohibited, so additionally
>>> allowing trusted keys won't break backwards compatibility.
>>>
>>> Signed-off-by: Ahmad Fatoum <a.fatoum@xxxxxxxxxxxxxx>
>>> ---
>>> Tested with:
>>> https://github.com/google/fscryptctl/pull/23
>>> -	if (key->type != &key_type_fscrypt_provisioning)
>>> -		goto bad_key;
>>> -	payload = key->payload.data[0];
>>> +	if (key->type == &key_type_fscrypt_provisioning) {
>>
>> Why does fscrypt have own key type, and does not extend 'encrypted' with a
>> new format [*]?
> 
> See the commit[1] adding it for more information. TL;DR:
> 
> fscrypt maintainers would've preferred keys to be associated with
> a "domain". So an encrypted key generated for fscrypt use couldn't be reused
> for e.g. dm-crypt. They are wary of fscrypt users being more exposed if their
> keys can be used with weaker ciphers via other kernel functionality that could
> be used to extract information about the raw key material.
> 
> Eric also mentioned dislike of the possibility of rooting encrypted keys to
> user keys. v2 is only restricted to v2, so we didn't discuss this further.

Typo: v2 (of my series) is only restricted to s/v2/trusted keys/

> 
> Restricting the key to fscrypt-only precludes this reuse.
> 
> My commit makes no attempts in changing that. It just adds a new way to pass
> raw key material into fscrypt. For more information, see the commit[1] adding
> that key type.
> 
>> [*] https://www.kernel.org/doc/html/v5.13/security/keys/trusted-encrypted.html
> 
> [1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=93edd392ca
> 
> Cheers,
> Ahmad
> 


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