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Date: Mon, 28 Jan 2013 16:54:06 +0200
From: "Kasatkin, Dmitry" <dmitry.kasatkin@...el.com>
To: Vivek Goyal <vgoyal@...hat.com>
Cc: Mimi Zohar <zohar@...ux.vnet.ibm.com>, dhowells@...hat.com,
jmorris@...ei.org, linux-security-module@...r.kernel.org,
linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC 1/1] ima: digital signature verification using asymmetric keys
On Fri, Jan 25, 2013 at 11:01 PM, Vivek Goyal <vgoyal@...hat.com> wrote:
> 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.
Hi,
Yes, currently it is possible to use only single configured algorithm, which is
in generally enough. Consider it like a policy.
Soon it will be a patch which allows to use any hash algorithms, supported by
asymmetric key verification API.
- Dmitry
>
> 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@...ux.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@...el.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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