linux-kernel - Re: sign-file and detached PKCS#7 firmware signatures

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Date:	Tue, 19 May 2015 01:13:04 +0200
From:	"Luis R. Rodriguez" <mcgrof@...e.com>
To:	David Howells <dhowells@...hat.com>
Cc:	rusty@...tcorp.com.au, mmarek@...e.cz, mjg59@...f.ucam.org,
	keyrings@...ux-nfs.org, dmitry.kasatkin@...il.com,
	linux-kernel@...r.kernel.org, seth.forshee@...onical.com,
	linux-security-module@...r.kernel.org, dwmw2@...radead.org
Subject: Re: sign-file and detached PKCS#7 firmware signatures

On Fri, May 15, 2015 at 08:07:55PM +0100, David Howells wrote:
> Hi Luis,
> 
> As David Woodhouse pointed out to me, you don't need sign-file if you're just
> going to create a detached PKCS#7 message as your signature.  You can just use
> "openssl smime" directly.
> 
> The reason that sign-file is needed for module signing is that the signature
> is added to the module with a little bit of metadata to indicate its presence
> - but if you're having detached signatures, that isn't relevant.
> 
> You can do this with two steps:
> 
>  (1) Require that an X.509 certificate is made available to the kernel to
>      provide the public key.  One way to do this is to convert it to DER form
>      and place it in the source directory as <name>.x509 when you build the
>      kernel.

OK.

>  (2) Document that to produce a signature for a firmware blob, you just run
>      the following command:
> 
> 		openssl smime -sign \
> 		 -in $FIRMWARE_BLOB_NAME \
> 		 -outform DER \
> 		 -inkey $PRIVATE_KEY_FILE_IN_PEM_FORM \
> 		 -signer $X509_CERT_FILE_IN_PEM_FORM \
> 		 -nocerts \
> 		 -md $DIGEST_ALGORITHM \

There's a missing -binary argument here, other than that this works fine.

> 		 >$PKCS7_MESSAGE_FILE_IN_DER_FORM

I however cannot figure out how to use openssl to verify this signature.

>      Note that if you have crypto hardware available that openssl can use, you
>      can do that in this command.
> 
> 
> To summarise, what you have to present to the kernel is the following:
> 
>  (A) A DER-encoded X.509 certificate containing the public key.
> 
>  (B) A DER-encoded PKCS#7 message containing the signatures.
> 
>  (C) A binary blob that is the detached data for the PKCS#7 message.

Will respin.

 Lui
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