Re: [PATCH 0/3] add support for metadata encryption to F2FS

[Eric Biggers <ebiggers@kernel.org>]

On Wed, Oct 07, 2020 at 10:05:00PM +0000, Satya Tangirala wrote:
> > I notice this is missing the step I suggested to include the metadata encryption
> > key in the HKDF application-specific info string when deriving subkeys from the
> > fscrypt master keys.
> > 
> > The same effect could also be achieved by adding an additional level to the key
> > hierarchy: each HKDF key would be derived from a fscrypt master key and the
> > metadata encryption key.
> > 
> > We need one of those, to guarantee that the file contents encryption is at least
> > as strong as the "metadata encryption".
> >
> Yes - I didn't get around to that in the first version, but I'll add
> that too in the next version. I was going to go with the first approach
> before I saw your comment - is there one method you'd recommend going
> with over the other?

I'm not entirely sure, but I'm now leaning towards the second approach because
it would avoid adding additional work (another SHA-512 block) to all later key
derivations.  Also it would avoid having to add a super_block argument to
fscrypt_hkdf_expand().  But please ask Paul Crowley for his suggestion too.

Here's a quick untested patch to consider:

diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h
index dca254590a70..67f8ba3098d3 100644
--- a/fs/crypto/fscrypt_private.h
+++ b/fs/crypto/fscrypt_private.h
@@ -319,6 +319,7 @@ int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
 #define HKDF_CONTEXT_DIRHASH_KEY	5 /* info=file_nonce		*/
 #define HKDF_CONTEXT_IV_INO_LBLK_32_KEY	6 /* info=mode_num||fs_uuid	*/
 #define HKDF_CONTEXT_INODE_HASH_KEY	7 /* info=<empty>		*/
+#define HKDF_CONTEXT_MIX_METADATA_KEY	8 /* info=metadata_key		*/
 
 int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context,
 			const u8 *info, unsigned int infolen,
@@ -600,6 +601,20 @@ int fscrypt_setup_v1_file_key(struct fscrypt_info *ci,
 
 int fscrypt_setup_v1_file_key_via_subscribed_keyrings(struct fscrypt_info *ci);
 
+/* metadata_crypt.c */
+
+#ifdef CONFIG_FS_ENCRYPTION_METADATA
+int fscrypt_mix_in_metadata_key(struct super_block *sb,
+				struct fscrypt_master_key_secret *secret);
+#else
+static inline int
+fscrypt_mix_in_metadata_key(struct super_block *sb,
+			    struct fscrypt_master_key_secret *secret)
+{
+	return 0;
+}
+#endif
+
 /* policy.c */
 
 bool fscrypt_policies_equal(const union fscrypt_policy *policy1,
diff --git a/fs/crypto/hkdf.c b/fs/crypto/hkdf.c
index 0cba7928446d..61d1f0aa802e 100644
--- a/fs/crypto/hkdf.c
+++ b/fs/crypto/hkdf.c
@@ -174,4 +174,5 @@ int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context,
 void fscrypt_destroy_hkdf(struct fscrypt_hkdf *hkdf)
 {
 	crypto_free_shash(hkdf->hmac_tfm);
+	hkdf->hmac_tfm = NULL;
 }
diff --git a/fs/crypto/keyring.c b/fs/crypto/keyring.c
index e74f239c4428..43453a7f77b1 100644
--- a/fs/crypto/keyring.c
+++ b/fs/crypto/keyring.c
@@ -494,6 +494,10 @@ static int add_master_key(struct super_block *sb,
 		 */
 		memzero_explicit(secret->raw, secret->size);
 
+		err = fscrypt_mix_in_metadata_key(sb, secret);
+		if (err)
+			return err;
+
 		/* Calculate the key identifier */
 		err = fscrypt_hkdf_expand(&secret->hkdf,
 					  HKDF_CONTEXT_KEY_IDENTIFIER, NULL, 0,
diff --git a/fs/crypto/metadata_crypt.c b/fs/crypto/metadata_crypt.c
index 5e16df130509..233e68c35504 100644
--- a/fs/crypto/metadata_crypt.c
+++ b/fs/crypto/metadata_crypt.c
@@ -13,6 +13,32 @@
 
 #include "fscrypt_private.h"
 
+/* TODO: add comment */
+int fscrypt_mix_in_metadata_key(struct super_block *sb,
+				struct fscrypt_master_key_secret *secret)
+{
+	u8 real_key[FSCRYPT_MAX_KEY_SIZE];
+	int err;
+
+	if (WARN_ON(secret->size > sizeof(real_key)))
+		return -EINVAL;
+
+	if (!sb->s_metadata_key)
+		return 0;
+
+	err = fscrypt_hkdf_expand(&secret->hkdf, HKDF_CONTEXT_MIX_METADATA_KEY,
+				  sb->s_metadata_key->raw,
+				  sb->s_metadata_key->size,
+				  real_key, secret->size);
+	if (err)
+		return err;
+
+	fscrypt_destroy_hkdf(&secret->hkdf);
+	err = fscrypt_init_hkdf(&secret->hkdf, real_key, secret->size);
+	memzero_explicit(real_key, secret->size);
+	return err;
+}
+
 /* TODO: mostly copied from keysetup_v1.c - maybe refactor this function */
 static int fscrypt_metadata_get_key_from_id(const char *prefix,
 					    char *descriptor_hex,