MODSIGN: Implement module signature checking

Check the signature on the module against the keys compiled into the kernel or
available in a hardware key store.

Currently, only RSA keys are supported - though that's easy enough to change,
and the signature is expected to contain raw components (so not a PGP or
PKCS#7 formatted blob).

The signature blob is expected to consist of the following pieces in order:

 (1) The binary identifier for the key.  This is expected to match the
     SubjectKeyIdentifier from an X.509 certificate.  Only X.509 type
     identifiers are currently supported.

 (2) The signature data, consisting of a series of MPIs in which each is in
     the format of a 2-byte BE word sizes followed by the content data.

 (3) A 12 byte information block of the form:

	struct module_signature {
		enum pkey_algo		algo : 8;
		enum pkey_hash_algo	hash : 8;
		enum pkey_id_type	id_type : 8;
		u8			__pad;
		__be32			id_length;
		__be32			sig_length;
	};

     The three enums are defined in crypto/public_key.h.

     'algo' contains the public-key algorithm identifier (0->DSA, 1->RSA).

     'hash' contains the digest algorithm identifier (0->MD4, 1->MD5, 2->SHA1,
      etc.).

     'id_type' contains the public-key identifier type (0->PGP, 1->X.509).

     '__pad' should be 0.

     'id_length' should contain in the binary identifier length in BE form.

     'sig_length' should contain in the signature data length in BE form.

     The lengths are in BE order rather than CPU order to make dealing with
     cross-compilation easier.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (minor Kconfig fix)

1 files changed, 221 insertions, 1 deletions

diff --git a/kernel/module_signing.c b/kernel/module_signing.c
index 499728aecaf..6b09f6983ac 100644
--- a/kernel/module_signing.c
+++ b/kernel/module_signing.c
@@ -11,13 +11,233 @@
 
 #include <linux/kernel.h>
 #include <linux/err.h>
+#include <crypto/public_key.h>
+#include <crypto/hash.h>
+#include <keys/asymmetric-type.h>
 #include "module-internal.h"
 
 /*
+ * Module signature information block.
+ *
+ * The constituents of the signature section are, in order:
+ *
+ *      - Signer's name
+ *      - Key identifier
+ *      - Signature data
+ *      - Information block
+ */
+struct module_signature {
+        enum pkey_algo          algo : 8;       /* Public-key crypto algorithm */
+        enum pkey_hash_algo     hash : 8;       /* Digest algorithm */
+        enum pkey_id_type       id_type : 8;    /* Key identifier type */
+        u8                      signer_len;     /* Length of signer's name */
+        u8                      key_id_len;     /* Length of key identifier */
+        u8                      __pad[3];
+        __be32                  sig_len;        /* Length of signature data */
+};
+
+/*
+ * Digest the module contents.
+ */
+static struct public_key_signature *mod_make_digest(enum pkey_hash_algo hash,
+                                                    const void *mod,
+                                                    unsigned long modlen)
+{
+        struct public_key_signature *pks;
+        struct crypto_shash *tfm;
+        struct shash_desc *desc;
+        size_t digest_size, desc_size;
+        int ret;
+
+        pr_devel("==>%s()\n", __func__);
+        
+        /* Allocate the hashing algorithm we're going to need and find out how
+         * big the hash operational data will be.
+         */
+        tfm = crypto_alloc_shash(pkey_hash_algo[hash], 0, 0);
+        if (IS_ERR(tfm))
+                return (PTR_ERR(tfm) == -ENOENT) ? ERR_PTR(-ENOPKG) : ERR_CAST(tfm);
+
+        desc_size = crypto_shash_descsize(tfm) + sizeof(*desc);
+        digest_size = crypto_shash_digestsize(tfm);
+
+        /* We allocate the hash operational data storage on the end of our
+         * context data and the digest output buffer on the end of that.
+         */
+        ret = -ENOMEM;
+        pks = kzalloc(digest_size + sizeof(*pks) + desc_size, GFP_KERNEL);
+        if (!pks)
+                goto error_no_pks;
+
+        pks->pkey_hash_algo     = hash;
+        pks->digest             = (u8 *)pks + sizeof(*pks) + desc_size;
+        pks->digest_size        = digest_size;
+
+        desc = (void *)pks + sizeof(*pks);
+        desc->tfm   = tfm;
+        desc->flags = CRYPTO_TFM_REQ_MAY_SLEEP;
+
+        ret = crypto_shash_init(desc);
+        if (ret < 0)
+                goto error;
+
+        ret = crypto_shash_finup(desc, mod, modlen, pks->digest);
+        if (ret < 0)
+                goto error;
+
+        crypto_free_shash(tfm);
+        pr_devel("<==%s() = ok\n", __func__);
+        return pks;
+
+error:
+        kfree(pks);
+error_no_pks:
+        crypto_free_shash(tfm);
+        pr_devel("<==%s() = %d\n", __func__, ret);
+        return ERR_PTR(ret);
+}
+
+/*
+ * Extract an MPI array from the signature data.  This represents the actual
+ * signature.  Each raw MPI is prefaced by a BE 2-byte value indicating the
+ * size of the MPI in bytes.
+ *
+ * RSA signatures only have one MPI, so currently we only read one.
+ */
+static int mod_extract_mpi_array(struct public_key_signature *pks,
+                                 const void *data, size_t len)
+{
+        size_t nbytes;
+        MPI mpi;
+
+        if (len < 3)
+                return -EBADMSG;
+        nbytes = ((const u8 *)data)[0] << 8 | ((const u8 *)data)[1];
+        data += 2;
+        len -= 2;
+        if (len != nbytes)
+                return -EBADMSG;
+
+        mpi = mpi_read_raw_data(data, nbytes);
+        if (!mpi)
+                return -ENOMEM;
+        pks->mpi[0] = mpi;
+        pks->nr_mpi = 1;
+        return 0;
+}
+
+/*
+ * Request an asymmetric key.
+ */
+static struct key *request_asymmetric_key(const char *signer, size_t signer_len,
+                                          const u8 *key_id, size_t key_id_len)
+{
+        key_ref_t key;
+        size_t i;
+        char *id, *q;
+
+        pr_devel("==>%s(,%zu,,%zu)\n", __func__, signer_len, key_id_len);
+
+        /* Construct an identifier. */
+        id = kmalloc(signer_len + 2 + key_id_len * 2 + 1, GFP_KERNEL);
+        if (!id)
+                return ERR_PTR(-ENOKEY);
+
+        memcpy(id, signer, signer_len);
+
+        q = id + signer_len;
+        *q++ = ':';
+        *q++ = ' ';
+        for (i = 0; i < key_id_len; i++) {
+                *q++ = hex_asc[*key_id >> 4];
+                *q++ = hex_asc[*key_id++ & 0x0f];
+        }
+
+        *q = 0;
+
+        pr_debug("Look up: \"%s\"\n", id);
+
+        key = keyring_search(make_key_ref(modsign_keyring, 1),
+                             &key_type_asymmetric, id);
+        if (IS_ERR(key))
+                pr_warn("Request for unknown module key '%s' err %ld\n",
+                        id, PTR_ERR(key));
+        kfree(id);
+
+        if (IS_ERR(key)) {
+                switch (PTR_ERR(key)) {
+                        /* Hide some search errors */
+                case -EACCES:
+                case -ENOTDIR:
+                case -EAGAIN:
+                        return ERR_PTR(-ENOKEY);
+                default:
+                        return ERR_CAST(key);
+                }
+        }
+
+        pr_devel("<==%s() = 0 [%x]\n", __func__, key_serial(key_ref_to_ptr(key)));
+        return key_ref_to_ptr(key);
+}
+
+/*
  * Verify the signature on a module.
  */
 int mod_verify_sig(const void *mod, unsigned long modlen,
                    const void *sig, unsigned long siglen)
 {
-        return -ENOKEY;
+        struct public_key_signature *pks;
+        struct module_signature ms;
+        struct key *key;
+        size_t sig_len;
+        int ret;
+
+        pr_devel("==>%s(,%lu,,%lu,)\n", __func__, modlen, siglen);
+
+        if (siglen <= sizeof(ms))
+                return -EBADMSG;
+
+        memcpy(&ms, sig + (siglen - sizeof(ms)), sizeof(ms));
+        siglen -= sizeof(ms);
+
+        sig_len = be32_to_cpu(ms.sig_len);
+        if (sig_len >= siglen ||
+            siglen - sig_len != (size_t)ms.signer_len + ms.key_id_len)
+                return -EBADMSG;
+
+        /* For the moment, only support RSA and X.509 identifiers */
+        if (ms.algo != PKEY_ALGO_RSA ||
+            ms.id_type != PKEY_ID_X509)
+                return -ENOPKG;
+
+        if (ms.hash >= PKEY_HASH__LAST ||
+            !pkey_hash_algo[ms.hash])
+                return -ENOPKG;
+
+        key = request_asymmetric_key(sig, ms.signer_len,
+                                     sig + ms.signer_len, ms.key_id_len);
+        if (IS_ERR(key))
+                return PTR_ERR(key);
+
+        pks = mod_make_digest(ms.hash, mod, modlen);
+        if (IS_ERR(pks)) {
+                ret = PTR_ERR(pks);
+                goto error_put_key;
+        }
+
+        ret = mod_extract_mpi_array(pks, sig + ms.signer_len + ms.key_id_len,
+                                    sig_len);
+        if (ret < 0)
+                goto error_free_pks;
+
+        ret = verify_signature(key, pks);
+        pr_devel("verify_signature() = %d\n", ret);
+
+error_free_pks:
+        mpi_free(pks->rsa.s);
+        kfree(pks);
+error_put_key:
+        key_put(key);
+        pr_devel("<==%s() = %d\n", __func__, ret);
+        return ret;     
 }