linux-kernel - Re: [PATCH] KEYS: encrypted: fix key instantiation with user-provided data

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Message-ID: <aac62bfc-2425-ffeb-1c49-e0963bdbfa99@vosn.de>
Date:   Wed, 5 Oct 2022 12:04:22 +0200 (CEST)
From:   Nikolaus Voss <nv@...n.de>
To:     Mimi Zohar <zohar@...ux.ibm.com>
cc:     David Howells <dhowells@...hat.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        linux-integrity@...r.kernel.org, keyrings@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, Yael Tzur <yaelt@...gle.com>
Subject: Re: [PATCH] KEYS: encrypted: fix key instantiation with user-provided
 data

On Wed, 28 Sep 2022, Mimi Zohar wrote:
> On Wed, 2022-09-28 at 14:08 +0200, Nikolaus Voss wrote:
>> On Wed, 21 Sep 2022, Mimi Zohar wrote:
>>> On Wed, 2022-09-21 at 09:24 +0200, Nikolaus Voss wrote:
>>>> On Tue, 20 Sep 2022, Mimi Zohar wrote:
>>>>> On Tue, 2022-09-20 at 18:23 +0200, Nikolaus Voss wrote:
>>>>>> On Tue, 20 Sep 2022, Mimi Zohar wrote:
>>>>>>> On Fri, 2022-09-16 at 07:45 +0200, Nikolaus Voss wrote:
>>>>>>>> Commit cd3bc044af48 ("KEYS: encrypted: Instantiate key with user-provided
>>>>>>>> decrypted data") added key instantiation with user provided decrypted data.
>>>>>>>> The user data is hex-ascii-encoded but was just memcpy'ed to the binary buffer.
>>>>>>>> Fix this to use hex2bin instead.
>>>>>>>
>>>>>>> Thanks, Nikolaus.  We iterated a number of times over what would be the
>>>>>>> safest userspace input.  One of the last changes was that the key data
>>>>>>> should be hex-ascii-encoded.  Unfortunately, the LTP
>>>>>>> testcases/kernel/syscalls/keyctl09.c example isn't hex-ascii-encoded
>>>>>>> and the example in Documentation/security/keys/trusted-encrypted.rst
>>>>>>> just cat's a file.  Both expect the length to be the length of the
>>>>>>> userspace provided data.   With this patch, when hex2bin() fails, there
>>>>>>> is no explanation.
>>>>>>
>>>>>> That's true. But it's true for all occurrences of hex2bin() in this file.
>>>>>> I could pr_err() an explanation, improve the trusted-encrypted.rst example
>>>>>> and respin the patch. Should I, or do you have another suggestion?
>>>>>
>>>>>> I wasn't aware of keyctl09.c, but quickly looking into it, the user data
>>>>>> _is_ hex-ascii-encoded, only the length is "wrong": Imho, the specified
>>>>>> length should be the binary length as this is consistent with key-length
>>>>>> specs in other cases (e.g. when loading the key from a blob).
>>>>>> keyctl09.c could be easy to fix, if only the length is modified. Should
>>>>>> I propose a patch? What is the correct/appropriate workflow there?
>>>>>
>>>>> I'm concerned that this change breaks existing encrypted keys created
>>>>> with user-provided data.  Otherwise I'm fine with your suggestion.
>>>>
>>>> Ok, but this change does not touch the hex-ascii format of encrypted key
>>>> blobs?
>>>
>>> True, but any persistent data based on this key would be affected.
>>
>> Persistent data is stored encypted with e.g. the master key in hex-ascii
>> already and should not be affected. Only persistent data stored
>> unencrypted is affected, but the encrypted-keys stuff is just about
>> avoiding that. Or do I still misunderstand something?
>
> Perhaps an existing encrypted key usage example would help clarify what
> is meant by persistent data.  The two original encrypted key usages are
> the EVM HMAC key and ecryptfs.  The EVM key is an encrypted key used to
> calculate the EVM HMAC, which is stored in security.evm.  In that
> scenario, the persistent data would be the data stored in security.evm.
>
> Would this patch break existing kernel/application persistent data
> based on encrypted keys created with user-provided data?

As far as I can tell, it does not.

Niko