linux-kernel - Re: [PATCH 00/14] KEYS: Add support for PGP keys and signatures

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Message-ID: <ab3d2bda-a704-f5d3-adee-e52b7d0a4641@maciej.szmigiero.name>
Date:   Wed, 12 Jan 2022 21:15:38 +0100
From:   "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
To:     Roberto Sassu <roberto.sassu@...wei.com>
Cc:     "keyrings@...r.kernel.org" <keyrings@...r.kernel.org>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "linux-fscrypt@...r.kernel.org" <linux-fscrypt@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "zohar@...ux.ibm.com" <zohar@...ux.ibm.com>,
        "ebiggers@...nel.org" <ebiggers@...nel.org>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>,
        "herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
        "davem@...emloft.net" <davem@...emloft.net>
Subject: Re: [PATCH 00/14] KEYS: Add support for PGP keys and signatures

On 12.01.2022 10:16, Roberto Sassu wrote:
>> From: Maciej S. Szmigiero [mailto:mail@...iej.szmigiero.name]
>> Sent: Tuesday, January 11, 2022 9:33 PM
>> On 11.01.2022 19:03, Roberto Sassu wrote:
>>> Support for PGP keys and signatures was proposed by David long time ago,
>>> before the decision of using PKCS#7 for kernel modules signatures
>>> verification was made. After that, there has been not enough interest to
>>> support PGP too.
>>>
>>> Lately, when discussing a proposal of introducing fsverity signatures in
>>> Fedora [1], developers expressed their preference on not having a separate
>>> key for signing, which would complicate the management of the distribution.
>>> They would be more in favor of using the same PGP key, currently used for
>>> signing RPM headers, also for file-based signatures (not only fsverity, but
>>> also IMA ones).
>>
>> Aren't PGP keys simply RSA / ECC / EdDSA keys with additional metadata?
>> Can't they be unwrapped from their (complex) PGP format in userspace and
>> loaded raw into the kernel, in a similar way as they are sometimes used
>> for SSH authentication?
> 
> Probably, this would be possible by introducing a new asymmetric
> key subtype parsing PGP keys and signatures in a more simple format,
> after conversion by user space. But still, a parser would be required.
> To be honest, I would prefer to implement (actually David did) a
> parser following an RFC, than developing a new one.

A parser in userspace is preferred to one in kernel since if there is
a bug somewhere its consequences are much less severe.
And experience shows that parsers are especially prone to bugs.
A userspace implementation can also be tightly sandboxed for extra
security.

There are many existing OpenPGP parsing libraries to choose from.

>> This will save us from having to add complex parsers (a well-known source
>> of bugs) into the kernel - I guess there aren't any plans to add an
>> in-kernel PGP Web of Trust implementation.
> 
> I extensively tested the implementation with an ad-hoc fault injector,
> to see if the code can correctly handle errors. I also developed a
> fuzzer to corrupt the data before it is read by the kernel. Finally,
> I checked that there are not memory leaks. But I agree, there could
> still be bugs.
> 
> If you mean that a key can be added to the kernel if is vouched for
> by another key in the built-in keyring, I actually implemented this
> (was missing in the original implementation). Some keyrings, e.g. .ima,
> have this restriction.
> 
> The way this works is that, whenever you add a PGP key to the
> kernel, the parser takes not only the public key and the user ID,
> but also its signature by the same or another PGP key.
> 
> The signature is verified when the key is added to the keyring
> with that restriction, and only if the verification is successful
> the key can be added.

I understand but it would be great to make use as much as possible of
the existing in-kernel signature verification mechanisms.

> Roberto

Thanks,
Maciej