Re: [RFC 1/1] ima: digital signature verification using asymmetric keys

[Vivek Goyal <vgoyal@redhat.com>]

Hi,

I am trying to read and understand IMA code. How does digital signature
mechanism work. 

IIUC, evmctl will install a file's signature in security.ima. And later
process_measurement() will do following.

Calculate digest of file in ima_collect_measurement() and then
ima_appraise_measurement() actually compares signatuer against the
digest.

If yes, ima_collect_measurement() always calculates digest either using
md5/sha1 but signatures might have used sha256 or something else. So
how does it work. What am I missing.

Thanks
Vivek

On Wed, Jan 23, 2013 at 11:03:39AM +0200, Kasatkin, Dmitry wrote:
> On Wed, Jan 23, 2013 at 12:53 AM, Mimi Zohar <zohar@linux.vnet.ibm.com> wrote:
> > On Tue, 2013-01-15 at 12:34 +0200, Dmitry Kasatkin wrote:
> >> Asymmetric keys were introduced in linux-3.7 to verify the signature on
> >> signed kernel modules.  The asymmetric keys infrastructure abstracts the
> >> signature verification from the crypto details.  This patch adds IMA/EVM
> >> signature verification using asymmetric keys.  Support for additional
> >> signature verification methods can now be delegated to the asymmetric
> >> key infrastructure.
> >>
> >> Signed-off-by: Dmitry Kasatkin <dmitry.kasatkin@intel.com>
> >> ---
> >>  security/integrity/Kconfig  |   12 +++++
> >>  security/integrity/digsig.c |  103 ++++++++++++++++++++++++++++++++++++++++++-
> >>  2 files changed, 114 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/security/integrity/Kconfig b/security/integrity/Kconfig
> >> index 5bd1cc1..19c4187 100644
> >> --- a/security/integrity/Kconfig
> >> +++ b/security/integrity/Kconfig
> >> @@ -17,5 +17,17 @@ config INTEGRITY_SIGNATURE
> >>         This is useful for evm and module keyrings, when keys are
> >>         usually only added from initramfs.
> >>
> >> +config INTEGRITY_ASYMMETRIC_KEYS
> >> +     boolean "Digital signature verification using asymmetric keys"
> >> +     depends on INTEGRITY_SIGNATURE
> >> +     default n
> >> +        select ASYMMETRIC_KEY_TYPE
> >> +        select ASYMMETRIC_PUBLIC_KEY_SUBTYPE
> >> +        select PUBLIC_KEY_ALGO_RSA
> >> +        select X509_CERTIFICATE_PARSER
> >> +     help
> >> +       This option enables digital signature verification support
> >> +       using asymmetric keys.
> >> +
> >>  source security/integrity/ima/Kconfig
> >>  source security/integrity/evm/Kconfig
> >> diff --git a/security/integrity/digsig.c b/security/integrity/digsig.c
> >> index 2dc167d..1896537 100644
> >> --- a/security/integrity/digsig.c
> >> +++ b/security/integrity/digsig.c
> >> @@ -15,10 +15,22 @@
> >>  #include <linux/err.h>
> >>  #include <linux/rbtree.h>
> >>  #include <linux/key-type.h>
> >> +#include <crypto/public_key.h>
> >> +#include <keys/asymmetric-type.h>
> >>  #include <linux/digsig.h>
> >>
> >>  #include "integrity.h"
> >>
> >> +/*
> >> + * signature format v2 - for using with asymmetric keys
> >> + */
> >> +struct signature_v2_hdr {
> >> +     uint8_t version;        /* signature format version */
> >> +     uint8_t hash_algo;      /* Digest algorithm [enum pkey_hash_algo] */
> >> +     uint8_t keyid[8];       /* IMA key identifier - not X509/PGP specific*/
> >> +     uint8_t payload[0];     /* signature payload */
> >> +} __packed;
> >> +
> >>  static struct key *keyring[INTEGRITY_KEYRING_MAX];
> >>
> >>  static const char *keyring_name[INTEGRITY_KEYRING_MAX] = {
> >> @@ -27,6 +39,91 @@ static const char *keyring_name[INTEGRITY_KEYRING_MAX] = {
> >>       "_ima",
> >>  };
> >>
> >> +#ifdef CONFIG_INTEGRITY_ASYMMETRIC_KEYS
> >> +
> >> +/*
> >> + * Request an asymmetric key.
> >> + */
> >> +static struct key *request_asymmetric_key(struct key *keyring, uint8_t *keyid)
> >> +{
> >> +     struct key *key;
> >> +     char name[20];
> >> +
> >> +     sprintf(name, "%llX", __be64_to_cpup((uint64_t *)keyid));
> >> +
> >> +     pr_debug("key search: \"%s\"\n", name);
> >> +
> >> +     if (keyring) {
> >> +             /* search in specific keyring */
> >> +             key_ref_t kref;
> >> +             kref = keyring_search(make_key_ref(keyring, 1),
> >> +                                   &key_type_asymmetric, name);
> >> +             if (IS_ERR(kref))
> >> +                     key = ERR_CAST(kref);
> >> +             else
> >> +                     key = key_ref_to_ptr(kref);
> >> +     } else {
> >> +             key = request_key(&key_type_asymmetric, name, NULL);
> >> +     }
> >> +
> >> +     if (IS_ERR(key)) {
> >> +             pr_warn("Request for unknown key '%s' err %ld\n",
> >> +                     name, PTR_ERR(key));
> >> +             switch (PTR_ERR(key)) {
> >> +                     /* Hide some search errors */
> >> +             case -EACCES:
> >> +             case -ENOTDIR:
> >> +             case -EAGAIN:
> >> +                     return ERR_PTR(-ENOKEY);
> >> +             default:
> >> +                     return key;
> >> +             }
> >> +     }
> >> +
> >> +     pr_debug("%s() = 0 [%x]\n", __func__, key_serial(key));
> >> +
> >> +     return key;
> >> +}
> >> +
> >> +static int asymmetric_verify(struct key *keyring, const char *sig,
> >> +                          size_t siglen, const char *data, int datalen)
> >> +{
> >> +     struct public_key_signature pks;
> >> +     struct signature_v2_hdr *hdr = (struct signature_v2_hdr *)sig;
> >> +     struct key *key;
> >> +     int ret = -ENOMEM;
> >> +
> >> +     if (siglen <= sizeof(*hdr))
> >> +             return -EBADMSG;
> >> +
> >> +     siglen -= sizeof(*hdr);
> >> +
> >> +     if (hdr->hash_algo >= PKEY_HASH__LAST)
> >> +             return -ENOPKG;
> >> +
> >> +     key = request_asymmetric_key(keyring, hdr->keyid);
> >> +     if (IS_ERR(key))
> >> +             return PTR_ERR(key);
> >> +
> >> +     memset(&pks, 0, sizeof(pks));
> >> +
> >> +     pks.pkey_hash_algo = hdr->hash_algo;
> >> +     pks.digest = (u8 *)data;
> >> +     pks.digest_size = datalen;
> >> +     pks.nr_mpi = 1;
> >> +     pks.rsa.s = mpi_read_from_buffer(hdr->payload, &siglen);
> >> +
> >> +     if (pks.rsa.s)
> >> +             ret = verify_signature(key, &pks);
> >> +
> >> +     mpi_free(pks.rsa.s);
> >> +     key_put(key);
> >> +     pr_debug("%s() = %d\n", __func__, ret);
> >> +     return ret;
> >> +}
> >> +
> >> +#endif /* CONFIG_INTEGRITY_ASYMMETRIC_KEYS */
> >> +
> >>  int integrity_digsig_verify(const unsigned int id, const char *sig, int siglen,
> >>                                       const char *digest, int digestlen)
> >>  {
> >> @@ -43,6 +140,10 @@ int integrity_digsig_verify(const unsigned int id, const char *sig, int siglen,
> >>                       return err;
> >>               }
> >>       }
> >> -
> >> +#ifdef CONFIG_INTEGRITY_ASYMMETRIC_KEYS
> >> +     if (sig[0] == 2)
> >> +             return asymmetric_verify(keyring[id], sig, siglen,
> >> +                                      digest, digestlen);
> >> +#endif
> >>       return digsig_verify(keyring[id], sig, siglen, digest, digestlen);
> >>  }
> >
> > Thanks Dmitry for the patch!  According to
> > Documentation/SubmittingPatches: section 2.2, #ifdefs are ugly.  I
> > realize this is a really small '.c' file, and doesn't really hurt
> > readability, but could you remove the ifdefs anyway?
> >
> 
> Will do it.
> 
> - Dmitry
> 
> > thanks,
> >
> > Mimi
> >
> >
> >
> > --
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