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Subject[PATCH 4/5] MODSIGN: checking the blacklisted hash before loading a kernel module
Date
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 <dhowells@redhat.com>
Cc: Josh Boyer <jwboyer@fedoraproject.org>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: "Lee, Chun-Yi" <jlee@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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