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

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Date:   Tue, 20 Sep 2022 18:53:49 -0400
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Nikolaus Voss <nv@...n.de>
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 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.

The LTP example decrypted data length is 32, but the minimum decrypted
data size is  20.  So it's a bit more than just changing the LTP
decrypted data size.   The modified LTP test should work on kernels
with and without this patch.

-- 
thanks,

Mimi