Date:	Wed, 18 Jan 2012 13:20:54 +0200
From:	"Kasatkin, Dmitry" <dmitry.kasatkin@...el.com>
To:	David Howells <dhowells@...hat.com>
Cc:	keyrings@...ux-nfs.org, linux-crypto@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org, zohar@...ux.vnet.ibm.com,
	arjan.van.de.ven@...el.com, alan.cox@...el.com
Subject: Re: [PATCH 08/21] KEYS: Add signature verification facility [ver #3]

On Fri, Dec 2, 2011 at 8:44 PM, David Howells <dhowells@...hat.com> wrote:
> Add a facility whereby a key subtype may be asked to verify a signature against
> the data it is purported to have signed.
>
> This adds four routines:
>
>  (1) struct crypto_key_verify_context *
>     verify_sig_begin(struct key *keyring, const void *sig, size_t siglen);
>
>     This sets up a verification context for the given signature using
>     information in that signature to select a key from the specified keyring
>     and to request a hash algorithm from the crypto layer.
>
>  (2) int verify_sig_add_data(struct crypto_key_verify_context *ctx,
>                             const void *data, size_t datalen);
>
>     Incrementally supply data to be signed.  May be called multiple times.
>
Hello,

It would also nice to have an API to supply pre-computed data hash.
For example IMA uses the same functionality to compute the hash of the
file content,
and then, based on security.ima type decided either verify it using just hash,
or use digital signature.
We could pass a hash as data. But may be we do not want to have extra
operation and compute hash over hash.

- Dmitry

>  (3) int verify_sig_end(struct crypto_key_verify_context *ctx,
>                        const void *sig, size_t siglen);
>
>     Complete the verification process and return the result.  -EKEYREJECTED
>     will indicate that the verification failed and 0 will indicate success.
>     Other errors are also possible.
>
>  (4) void verify_sig_cancel(struct crypto_key_verify_context *ctx);
>
>     Cancel the verification process.
>
> Signed-off-by: David Howells <dhowells@...hat.com>
> ---
>
>  Documentation/security/keys-crypto.txt |  101 +++++++++++++++++++++++++++++
>  include/keys/crypto-subtype.h          |   21 ++++++
>  include/keys/crypto-type.h             |    9 +++
>  security/keys/Makefile                 |    2 -
>  security/keys/crypto_verify.c          |  111 ++++++++++++++++++++++++++++++++
>  5 files changed, 243 insertions(+), 1 deletions(-)
>  create mode 100644 security/keys/crypto_verify.c
>
>
> diff --git a/Documentation/security/keys-crypto.txt b/Documentation/security/keys-crypto.txt
> index 97dee80..a964717 100644
> --- a/Documentation/security/keys-crypto.txt
> +++ b/Documentation/security/keys-crypto.txt
> @@ -7,6 +7,7 @@ Contents:
>   - Overview.
>   - Key identification.
>   - Accessing crypto keys.
> +    - Signature verification.
>   - Implementing crypto parsers.
>   - Implementing crypto subtypes.
>
> @@ -89,6 +90,65 @@ This gives access to the key type:
>        struct key_type key_type_crypto;
>
>
> +SIGNATURE VERIFICATION
> +----------------------
> +
> +The four operations that can perform cryptographic signature verification,
> +using one of a set of keys to provide the public key:
> +
> + (1) Begin verification procedure.
> +
> +       struct crypto_key_verify_context *
> +       verify_sig_begin(struct key *keyring, const void *sig, size_t siglen);
> +
> +     This function sets up a verification context from the information in the
> +     signature and looks for a suitable key in the keyring.  The signature blob
> +     must be presented again at the end of the procedure.  The keys will be
> +     checked against parameters in the signature, and if the matching one is
> +     not found then -ENOKEY will be returned.
> +
> +     The hashing algorithm, if such a thing applies, will be determined from
> +     information in the signature and the appropriate crypto module will be
> +     used.  -ENOPKG will be returned if the hash algorithm is unavailable.
> +
> +     The return value is an opaque pointer to be passed to the other functions,
> +     or a negative error code.
> +
> + (2) Indicate data to be verified.
> +
> +       int verify_sig_add_data(struct crypto_key_verify_context *ctx,
> +                               const void *data, size_t datalen);
> +
> +     This function is used to shovel data to the verification procedure so that
> +     it can load it into the hash, pass it to hardware or whatever is
> +     appropriate for the algorithm being employed.
> +
> +     The data is not canonicalised for the document type specified in the
> +     signature.  The caller must do that.
> +
> +     It will return 0 if successful and a negative error code if not.
> +
> + (3) Complete the verification process.
> +
> +       int verify_sig_end(struct crypto_key_verify_context *ctx,
> +                          const void *sig, size_t siglen);
> +
> +     This function performs the actual signature verification step and cleans
> +     up the resources allocated at the beginning.  The signature must be
> +     presented again as some of the data therein may need to be added to the
> +     internal hash.
> +
> +     It will return -EKEYREJECTED if the signature didn't match, 0 if
> +     successful and may return other errors as appropriate.
> +
> + (4) Cancel the verification process.
> +
> +       void verify_sig_cancel(struct crypto_key_verify_context *ctx);
> +
> +     This function cleans up the resources allocated at the beginning.  This is
> +     not necessary if verify_sig_end() was called.
> +
> +
>  ===========================
>  IMPLEMENTING CRYPTO PARSERS
>  ===========================
> @@ -96,6 +156,7 @@ IMPLEMENTING CRYPTO PARSERS
>  The crypto key type keeps a list of registered data parsers.  An example of
>  such a parser is one that parses OpenPGP packet formatted data [RFC 4880].
>
> +
>  During key instantiation each parser in the list is tried until one doesn't
>  return -EBADMSG.
>
> @@ -107,6 +168,8 @@ The parser definition structure looks like the following:
>
>                int (*instantiate)(struct key *key,
>                                   const void *data, size_t datalen);
> +               struct crypto_key_verify_context *(*verify_sig_begin)(
> +                       struct key *keyring, const u8 *sig, size_t siglen);
>        };
>
>  The owner and name fields should be set to the owning module and the name of
> @@ -135,6 +198,44 @@ but it is expected that at least one will be defined.
>      algorithm such as RSA and DSA this will likely be a printable hex version
>      of the key's fingerprint.
>
> + (2) verify_sig_begin().
> +
> +     This is similar in concept to the instantiate() function, except that it
> +     is given a signature blob to parse rather than a key data blob.
> +
> +     If the data format is not recognised, -EBADMSG should be returned.  If it
> +     is recognised, but the signature verification process cannot for some
> +     reason be set up, some other negative error code should be returned.
> +     -ENOKEY should be used to indicate that no matching key is available and
> +     -ENOPKG should be returned if the hash algorithm or the verification
> +     algorithm are unavailable.
> +
> +     If successful, the parser should allocate a verification context and embed
> +     the following struct in it:
> +
> +       struct crypto_key_verify_context {
> +               struct key *key;
> +               int (*add_data)(struct crypto_key_verify_context *ctx,
> +                               const void *data, size_t datalen);
> +               int (*end)(struct crypto_key_verify_context *ctx,
> +                          const u8 *sig, size_t siglen);
> +               void (*cancel)(struct crypto_key_verify_context *ctx);
> +       };
> +
> +     and return a pointer to this to the caller, who will then pass it to the
> +     verification operation wrappers described in the "Signature Verification"
> +     section.  The three operation pointers here correspond exactly to those
> +     wrappers and are all mandatory.  container_of() should be used to retrieve
> +     the actual context.
> +
> +     Note that the crypto key type retains a reference on the parser module for
> +     the lifetime of this context, though the operation pointers need not point
> +     into this module.
> +
> +     The parser should also record a pointer to the key selected and take a
> +     reference on that key with key_get().
> +
> +
>  Functions are provided to register and unregister parsers:
>
>        int register_crypto_key_parser(struct crypto_key_parser *parser);
> diff --git a/include/keys/crypto-subtype.h b/include/keys/crypto-subtype.h
> index fa87555..f2b927a 100644
> --- a/include/keys/crypto-subtype.h
> +++ b/include/keys/crypto-subtype.h
> @@ -20,6 +20,20 @@
>  extern struct key_type key_type_crypto;
>
>  /*
> + * Context base for signature verification methods.  Allocated by the subtype
> + * and presumably embedded in something appropriate.
> + */
> +struct crypto_key_verify_context {
> +       struct key *key;
> +       struct crypto_key_parser *parser;
> +       int (*add_data)(struct crypto_key_verify_context *ctx,
> +                       const void *data, size_t datalen);
> +       int (*end)(struct crypto_key_verify_context *ctx,
> +                  const u8 *sig, size_t siglen);
> +       void (*cancel)(struct crypto_key_verify_context *ctx);
> +};
> +
> +/*
>  * Keys of this type declare a subtype that indicates the handlers and
>  * capabilities.
>  */
> @@ -48,6 +62,13 @@ struct crypto_key_parser {
>         * Return EBADMSG if not recognised.
>         */
>        int (*instantiate)(struct key *key, const void *data, size_t datalen);
> +
> +       /* Attempt to recognise a signature blob and find a matching key.
> +        *
> +        * Return EBADMSG if not recognised.
> +        */
> +       struct crypto_key_verify_context *(*verify_sig_begin)(
> +               struct key *keyring, const u8 *sig, size_t siglen);
>  };
>
>  extern int register_crypto_key_parser(struct crypto_key_parser *);
> diff --git a/include/keys/crypto-type.h b/include/keys/crypto-type.h
> index 47c00c7..6b93366 100644
> --- a/include/keys/crypto-type.h
> +++ b/include/keys/crypto-type.h
> @@ -18,6 +18,15 @@
>
>  extern struct key_type key_type_crypto;
>
> +struct crypto_key_verify_context;
> +extern struct crypto_key_verify_context *verify_sig_begin(
> +       struct key *key, const void *sig, size_t siglen);
> +extern int verify_sig_add_data(struct crypto_key_verify_context *ctx,
> +                              const void *data, size_t datalen);
> +extern int verify_sig_end(struct crypto_key_verify_context *ctx,
> +                         const void *sig, size_t siglen);
> +extern void verify_sig_cancel(struct crypto_key_verify_context *ctx);
> +
>  /*
>  * The payload is at the discretion of the subtype.
>  */
> diff --git a/security/keys/Makefile b/security/keys/Makefile
> index 67fceaa..8462904 100644
> --- a/security/keys/Makefile
> +++ b/security/keys/Makefile
> @@ -26,4 +26,4 @@ obj-$(CONFIG_TRUSTED_KEYS) += trusted.o
>  obj-$(CONFIG_ENCRYPTED_KEYS) += encrypted-keys/
>  obj-$(CONFIG_CRYPTO_KEY_TYPE) += crypto_keys.o
>
> -crypto_keys-y := crypto_type.o
> +crypto_keys-y := crypto_type.o crypto_verify.o
> diff --git a/security/keys/crypto_verify.c b/security/keys/crypto_verify.c
> new file mode 100644
> index 0000000..65f734c
> --- /dev/null
> +++ b/security/keys/crypto_verify.c
> @@ -0,0 +1,111 @@
> +/* Signature verification with a crypto key
> + *
> + * Copyright (C) 2011 Red Hat, Inc. All Rights Reserved.
> + * Written by David Howells (dhowells@...hat.com)
> + *
> + * This program is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU General Public Licence
> + * as published by the Free Software Foundation; either version
> + * 2 of the Licence, or (at your option) any later version.
> + *
> + * See Documentation/security/keys-crypto.txt
> + */
> +
> +#include <keys/crypto-subtype.h>
> +#include <linux/module.h>
> +#include "crypto_keys.h"
> +
> +/**
> + * verify_sig_begin - Initiate the use of a crypto key to verify a signature
> + * @keyring: The public keys to verify against
> + * @sig: The signature data
> + * @siglen: The signature length
> + *
> + * Returns a context or an error.
> + */
> +struct crypto_key_verify_context *verify_sig_begin(
> +       struct key *keyring, const void *sig, size_t siglen)
> +{
> +       struct crypto_key_verify_context *ret;
> +       struct crypto_key_parser *parser;
> +
> +       pr_devel("==>%s()\n", __func__);
> +
> +       if (siglen == 0 || !sig)
> +               return ERR_PTR(-EINVAL);
> +
> +       down_read(&crypto_key_parsers_sem);
> +
> +       ret = ERR_PTR(-EBADMSG);
> +       list_for_each_entry(parser, &crypto_key_parsers, link) {
> +               if (parser->verify_sig_begin) {
> +                       if (!try_module_get(parser->owner))
> +                               continue;
> +
> +                       pr_debug("Trying parser '%s'\n", parser->name);
> +
> +                       ret = parser->verify_sig_begin(keyring, sig, siglen);
> +                       if (IS_ERR(ret))
> +                               module_put(parser->owner);
> +                       else
> +                               ret->parser = parser;
> +                       if (ret != ERR_PTR(-EBADMSG)) {
> +                               pr_debug("Parser recognised the format"
> +                                        " (ret %ld)\n",
> +                                        PTR_ERR(ret));
> +                               break;
> +                       }
> +               }
> +       }
> +
> +       up_read(&crypto_key_parsers_sem);
> +       pr_devel("<==%s() = %p\n", __func__, ret);
> +       return ret;
> +}
> +EXPORT_SYMBOL_GPL(verify_sig_begin);
> +
> +/**
> + * verify_sig_add_data - Incrementally provide data to be verified
> + * @ctx: The context from verify_sig_begin()
> + * @data: Data
> + * @datalen: The amount of @data
> + *
> + * This may be called multiple times.
> + */
> +int verify_sig_add_data(struct crypto_key_verify_context *ctx,
> +                       const void *data, size_t datalen)
> +{
> +       return ctx->add_data(ctx, data, datalen);
> +}
> +EXPORT_SYMBOL_GPL(verify_sig_add_data);
> +
> +/**
> + * verify_sig_end - Finalise signature verification and return result
> + * @ctx: The context from verify_sig_begin()
> + * @sig: The signature data
> + * @siglen: The signature length
> + */
> +int verify_sig_end(struct crypto_key_verify_context *ctx,
> +                  const void *sig, size_t siglen)
> +{
> +       struct crypto_key_parser *parser = ctx->parser;
> +       int ret;
> +
> +       ret = ctx->end(ctx, sig, siglen);
> +       module_put(parser->owner);
> +       return ret;
> +}
> +EXPORT_SYMBOL_GPL(verify_sig_end);
> +
> +/**
> + * verify_sig_end - Cancel signature verification
> + * @ctx: The context from verify_sig_begin()
> + */
> +void verify_sig_cancel(struct crypto_key_verify_context *ctx)
> +{
> +       struct crypto_key_parser *parser = ctx->parser;
> +
> +       ctx->cancel(ctx);
> +       module_put(parser->owner);
> +}
> +EXPORT_SYMBOL_GPL(verify_sig_cancel);
>
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