linux-kernel - Re: [PATCH RESEND] KEYS: trusted: Use ASN.1 encoded OID

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Date: Thu, 23 May 2024 16:54:02 +0300
From: "Jarkko Sakkinen" <jarkko@...nel.org>
To: "James Bottomley" <James.Bottomley@...senPartnership.com>,
 <linux-integrity@...r.kernel.org>
Cc: <keyrings@...r.kernel.org>, "David Woodhouse" <dwmw2@...radead.org>,
 "Eric Biggers" <ebiggers@...nel.org>, "Herbert Xu"
 <herbert@...dor.apana.org.au>, "David S. Miller" <davem@...emloft.net>,
 "Andrew Morton" <akpm@...ux-foundation.org>, "Mimi Zohar"
 <zohar@...ux.ibm.com>, "David Howells" <dhowells@...hat.com>, "Paul Moore"
 <paul@...l-moore.com>, "James Morris" <jmorris@...ei.org>, "Serge E.
 Hallyn" <serge@...lyn.com>, "open list:CRYPTO API"
 <linux-crypto@...r.kernel.org>, "open list" <linux-kernel@...r.kernel.org>,
 "open list:SECURITY SUBSYSTEM" <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH RESEND] KEYS: trusted: Use ASN.1 encoded OID

On Thu May 23, 2024 at 4:38 PM EEST, James Bottomley wrote:
> On Thu, 2024-05-23 at 16:19 +0300, Jarkko Sakkinen wrote:
> > There's no reason to encode OID_TPMSealedData at run-time, as it
> > never changes.
> > 
> > Replace it with the encoded version, which has exactly the same size:
> > 
> >         67 81 05 0A 01 05
> > 
> > Include OBJECT IDENTIFIER (0x06) tag and length as the epilogue so
> > that the OID can be simply copied to the blob.
>
> This is true, but if we're going to do this, we should expand the OID
> registry functions (in lib/oid_registry.c) to do something like
> encode_OID.  The registry already contains the hex above minus the two
> prefixes (which are easy to add).

Yes, I do agree with this idea, and I named variable the I named
it to make it obvious that generation is possible.

It would be best to have a single source, which could be just
a CSV file with entries like:

<Name>,<OID number>

And then in scripts/ there should be a script that takes this
source and generates oid_registry.gen.{h,c}. The existing
oid_registry.h should really just include oid_registry.gen.h
then to make this transparent change.

And then in the series where OID's are encoded per-subsystem
patch that takes pre-encoded OID into use.

Happy to review such patch set if it is pushed forward.

> > @ -51,8 +52,8 @@ static int tpm2_key_encode(struct
> > trusted_key_payload *payload,
> >         if (!scratch)
> >                 return -ENOMEM;
> >  
> > -       work = asn1_encode_oid(work, end_work, tpm2key_oid,
> > -                              asn1_oid_len(tpm2key_oid));
> > +       work = memcpy(work, OID_TPMSealedData_ASN1,
> > sizeof(OID_TPMSealedData_ASN1));
> > +       work += sizeof(OID_TPMSealedData_ASN1);
>
> You lost the actually fits check.  This is somewhat irrelevant for TPM
> keys because the OID is first in the structure and thus will never
> overflow, but it might matter for other uses.

Yep, it is irrelevant IMHO, there is 8 bytes, and also its location
never changes.

> James

BR, Jarkko