[PATCH 4/5] MODSIGN: checking the blacklisted hash before loading a kernel module

[Lee, Chun-Yi]

This patch adds the logic for checking the kernel module's hash
base on blacklist. The hash must be generated by sha256 and enrolled
to dbx/mokx.

For example:
        sha256sum sample.ko
        mokutil --mokx --import-hash $HASH_RESULT

Whether the signature on ko file is stripped or not, the hash can be
compared by kernel.

Cc: David Howells <dhowe...@redhat.com>
Cc: Josh Boyer <jwbo...@fedoraproject.org>
Cc: James Bottomley <james.bottom...@hansenpartnership.com>
Signed-off-by: "Lee, Chun-Yi" <j...@suse.com>
---
 kernel/module_signing.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 60 insertions(+), 2 deletions(-)

diff --git a/kernel/module_signing.c b/kernel/module_signing.c
index d3d6f95..d30ac74 100644
--- a/kernel/module_signing.c
+++ b/kernel/module_signing.c
@@ -11,9 +11,12 @@
 
 #include <linux/kernel.h>
 #include <linux/errno.h>
+#include <linux/module.h>
 #include <linux/string.h>
 #include <linux/verification.h>
 #include <crypto/public_key.h>
+#include <crypto/hash.h>
+#include <keys/system_keyring.h>
 #include "module-internal.h"
 
 enum pkey_id_type {
@@ -42,19 +45,67 @@ struct module_signature {
        __be32  sig_len;        /* Length of signature data */
 };
 
+static int mod_is_hash_blacklisted(const void *mod, size_t verifylen)
+{
+       struct crypto_shash *tfm;
+       struct shash_desc *desc;
+       size_t digest_size, desc_size;
+       u8 *digest;
+       int ret = 0;
+
+       tfm = crypto_alloc_shash("sha256", 0, 0);
+       if (IS_ERR(tfm))
+               goto error_return;
+
+       desc_size = crypto_shash_descsize(tfm) + sizeof(*desc);
+       digest_size = crypto_shash_digestsize(tfm);
+       digest = kzalloc(digest_size + desc_size, GFP_KERNEL);
+       if (!digest) {
+               pr_err("digest memory buffer allocate fail\n");
+               ret = -ENOMEM;
+               goto error_digest;
+       }
+       desc = (void *)digest + digest_size;
+       desc->tfm = tfm;
+       desc->flags = CRYPTO_TFM_REQ_MAY_SLEEP;
+       ret = crypto_shash_init(desc);
+       if (ret < 0)
+               goto error_shash;
+
+       ret = crypto_shash_finup(desc, mod, verifylen, digest);
+       if (ret < 0)
+               goto error_shash;
+
+       pr_debug("%ld digest: %*phN\n", verifylen, (int) digest_size, digest);
+
+       ret = is_hash_blacklisted(digest, digest_size, "bin");
+       if (ret == -EKEYREJECTED)
+               pr_err("Module hash %*phN is blacklisted\n",
+                      (int) digest_size, digest);
+
+error_shash:
+       kfree(digest);
+error_digest:
+       crypto_free_shash(tfm);
+error_return:
+       return ret;
+}
+
 /*
  * Verify the signature on a module.
  */
 int mod_verify_sig(const void *mod, unsigned long *_modlen)
 {
        struct module_signature ms;
-       size_t modlen = *_modlen, sig_len;
+       size_t modlen = *_modlen, sig_len, wholelen;
+       int ret;
 
        pr_devel("==>%s(,%zu)\n", __func__, modlen);
 
        if (modlen <= sizeof(ms))
                return -EBADMSG;
 
+       wholelen = modlen + sizeof(MODULE_SIG_STRING) - 1;
        memcpy(&ms, mod + (modlen - sizeof(ms)), sizeof(ms));
        modlen -= sizeof(ms);
 
@@ -80,7 +131,14 @@ int mod_verify_sig(const void *mod, unsigned long *_modlen)
                return -EBADMSG;
        }
 
-       return verify_pkcs7_signature(mod, modlen, mod + modlen, sig_len,
+       ret = verify_pkcs7_signature(mod, modlen, mod + modlen, sig_len,
                                      (void *)1UL, VERIFYING_MODULE_SIGNATURE,
                                      NULL, NULL);
+       pr_devel("verify_pkcs7_signature() = %d\n", ret);
+
+       /* checking hash of module is in blacklist */
+       if (!ret)
+               ret = mod_is_hash_blacklisted(mod, wholelen);
+
+       return ret;
 }
-- 
2.10.2

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