[v2,5/7] fscrypt: verify that the correct master key was supplied

Submitted by Eric Biggers on July 26, 2017, 6:19 p.m.

Details

Message ID 20170726181929.99880-6-ebiggers3@gmail.com
State New
Headers show

Commit Message

Eric Biggers July 26, 2017, 6:19 p.m.
From: Eric Biggers <ebiggers@google.com>

Currently, while a fscrypt master key is required to have a certain
description in the keyring, its payload is never verified to be correct.
While sufficient for well-behaved userspace, this is insecure in a
multi-user system where a user has been given only read-only access to
an encrypted file or directory.  Specifically, if an encrypted file or
directory does not yet have its key cached by the kernel, the first user
who accesses it can provide an arbitrary key in their own keyring, which
the kernel will then associate with the inode and use for read(),
write(), readdir(), etc. by other users as well.

Consequently, it's trivial for a user with *read-only* access to an
encrypted file or directory to make it appear as garbage to everyone.
Creative users might be able to accomplish more sophisticated attacks by
careful choice of the key, e.g. choosing a key causes certain bytes of
file contents to have certain values or that causes filenames containing
the '/' character to appear.

Solve the problem for v2 encryption policies by storing a "hash" of the
master encryption key in the encryption xattr and verifying it before
accepting the user-provided key.  We generate the "hash" using
HKDF-SHA512 by passing a distinct application-specific info string.
This produces a value which is cryptographically isolated and can be
stored in the clear without leaking any information about the master key
or any other derived keys (in a computational sense).  Reusing HKDF is
better than doing e.g. SHA-512(master_key) because it avoids passing the
same key into different cryptographic primitives.

We make the hash field 16 bytes long, as this should provide sufficient
collision and preimage resistance while not wasting too much space for
the encryption xattr.

Acked-by: Michael Halcrow <mhalcrow@google.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 fs/crypto/fscrypt_private.h |  4 ++++
 fs/crypto/keyinfo.c         | 46 +++++++++++++++++++++++++++++++++++++
 fs/crypto/policy.c          | 55 ++++++++++++++++++++++++++++++++++++---------
 3 files changed, 95 insertions(+), 10 deletions(-)

Patch hide | download patch | download mbox

diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h
index b5881cb26e8f..f9469a71cc29 100644
--- a/fs/crypto/fscrypt_private.h
+++ b/fs/crypto/fscrypt_private.h
@@ -94,6 +94,7 @@  fscrypt_valid_context_format(const struct fscrypt_context *ctx, int size)
  */
 struct fscrypt_master_key {
 	struct crypto_shash	*mk_hmac;
+	u8			mk_hash[FSCRYPT_KEY_HASH_SIZE];
 };
 
 /*
@@ -157,6 +158,9 @@  extern struct page *fscrypt_alloc_bounce_page(struct fscrypt_ctx *ctx,
 					      gfp_t gfp_flags);
 
 /* keyinfo.c */
+extern int fscrypt_compute_key_hash(const struct inode *inode,
+				    const struct fscrypt_policy *policy,
+				    u8 hash[FSCRYPT_KEY_HASH_SIZE]);
 extern void __exit fscrypt_essiv_cleanup(void);
 
 #endif /* _FSCRYPT_PRIVATE_H */
diff --git a/fs/crypto/keyinfo.c b/fs/crypto/keyinfo.c
index 3164e1c74196..a9514078cf9d 100644
--- a/fs/crypto/keyinfo.c
+++ b/fs/crypto/keyinfo.c
@@ -39,8 +39,11 @@  static struct crypto_shash *essiv_hash_tfm;
  *
  * Keys derived with different info strings are cryptographically isolated from
  * each other --- knowledge of one derived key doesn't reveal any others.
+ * (This property is particularly important for the derived key used as the
+ * "key hash", as that is stored in the clear.)
  */
 #define HKDF_CONTEXT_PER_FILE_KEY	1
+#define HKDF_CONTEXT_KEY_HASH		2
 
 /*
  * HKDF consists of two steps:
@@ -211,6 +214,12 @@  alloc_master_key(const struct fscrypt_key *payload)
 	err = crypto_shash_setkey(k->mk_hmac, prk, sizeof(prk));
 	if (err)
 		goto fail;
+
+	/* Calculate the "key hash" */
+	err = hkdf_expand(k->mk_hmac, HKDF_CONTEXT_KEY_HASH, NULL, 0,
+			  k->mk_hash, FSCRYPT_KEY_HASH_SIZE);
+	if (err)
+		goto fail;
 out:
 	memzero_explicit(prk, sizeof(prk));
 	return k;
@@ -536,6 +545,31 @@  void __exit fscrypt_essiv_cleanup(void)
 	crypto_free_shash(essiv_hash_tfm);
 }
 
+int fscrypt_compute_key_hash(const struct inode *inode,
+			     const struct fscrypt_policy *policy,
+			     u8 hash[FSCRYPT_KEY_HASH_SIZE])
+{
+	struct fscrypt_master_key *k;
+	unsigned int min_keysize;
+
+	/*
+	 * Require that the master key be long enough for both the
+	 * contents and filenames encryption modes.
+	 */
+	min_keysize =
+		max(available_modes[policy->contents_encryption_mode].keysize,
+		    available_modes[policy->filenames_encryption_mode].keysize);
+
+	k = load_master_key_from_keyring(inode, policy->master_key_descriptor,
+					 min_keysize);
+	if (IS_ERR(k))
+		return PTR_ERR(k);
+
+	memcpy(hash, k->mk_hash, FSCRYPT_KEY_HASH_SIZE);
+	put_master_key(k);
+	return 0;
+}
+
 int fscrypt_get_encryption_info(struct inode *inode)
 {
 	struct fscrypt_info *crypt_info;
@@ -612,6 +646,18 @@  int fscrypt_get_encryption_info(struct inode *inode)
 			goto out;
 		}
 
+		/*
+		 * Make sure the master key we found has the correct hash.
+		 * Buggy or malicious userspace may provide the wrong key.
+		 */
+		if (memcmp(crypt_info->ci_master_key->mk_hash, ctx.key_hash,
+			   FSCRYPT_KEY_HASH_SIZE)) {
+			pr_warn_ratelimited("fscrypt: wrong encryption key supplied for inode %lu\n",
+					    inode->i_ino);
+			res = -ENOKEY;
+			goto out;
+		}
+
 		res = derive_key_hkdf(crypt_info->ci_master_key, &ctx,
 				      derived_key, derived_keysize);
 	}
diff --git a/fs/crypto/policy.c b/fs/crypto/policy.c
index d1e58798da3c..c0a047559736 100644
--- a/fs/crypto/policy.c
+++ b/fs/crypto/policy.c
@@ -42,7 +42,8 @@  static u8 context_version_for_policy(const struct fscrypt_policy *policy)
  */
 static bool is_encryption_context_consistent_with_policy(
 				const struct fscrypt_context *ctx,
-				const struct fscrypt_policy *policy)
+				const struct fscrypt_policy *policy,
+				const u8 key_hash[FSCRYPT_KEY_HASH_SIZE])
 {
 	return (ctx->version == context_version_for_policy(policy)) &&
 		(memcmp(ctx->master_key_descriptor,
@@ -52,11 +53,14 @@  static bool is_encryption_context_consistent_with_policy(
 		(ctx->contents_encryption_mode ==
 		 policy->contents_encryption_mode) &&
 		(ctx->filenames_encryption_mode ==
-		 policy->filenames_encryption_mode);
+		 policy->filenames_encryption_mode) &&
+		(ctx->version == FSCRYPT_CONTEXT_V1 ||
+		 (memcmp(ctx->key_hash, key_hash, FSCRYPT_KEY_HASH_SIZE) == 0));
 }
 
 static int create_encryption_context_from_policy(struct inode *inode,
-				const struct fscrypt_policy *policy)
+				const struct fscrypt_policy *policy,
+				const u8 key_hash[FSCRYPT_KEY_HASH_SIZE])
 {
 	struct fscrypt_context ctx;
 
@@ -69,7 +73,7 @@  static int create_encryption_context_from_policy(struct inode *inode,
 	BUILD_BUG_ON(sizeof(ctx.nonce) != FS_KEY_DERIVATION_NONCE_SIZE);
 	get_random_bytes(ctx.nonce, FS_KEY_DERIVATION_NONCE_SIZE);
 	if (ctx.version != FSCRYPT_CONTEXT_V1)
-		memset(ctx.key_hash, 0, FSCRYPT_KEY_HASH_SIZE);
+		memcpy(ctx.key_hash, key_hash, FSCRYPT_KEY_HASH_SIZE);
 
 	return inode->i_sb->s_cop->set_context(inode, &ctx,
 					       fscrypt_context_size(&ctx),
@@ -82,6 +86,7 @@  int fscrypt_ioctl_set_policy(struct file *filp, const void __user *arg)
 	struct inode *inode = file_inode(filp);
 	int ret;
 	struct fscrypt_context ctx;
+	u8 key_hash[FSCRYPT_KEY_HASH_SIZE];
 
 	if (copy_from_user(&policy, arg, sizeof(policy)))
 		return -EFAULT;
@@ -100,6 +105,25 @@  int fscrypt_ioctl_set_policy(struct file *filp, const void __user *arg)
 	if (policy.flags & ~FS_POLICY_FLAGS_VALID)
 		return -EINVAL;
 
+	if (policy.version == FS_POLICY_VERSION_ORIGINAL) {
+		/*
+		 * Originally no key verification was implemented, which was
+		 * insufficient for scenarios where multiple users share
+		 * encrypted files.  The new encryption policy version fixes
+		 * this and also implements an improved key derivation function.
+		 * So as long as the key can be in the keyring at the time the
+		 * policy is set and compatibility with old kernels isn't
+		 * required, it's recommended to use the new policy version
+		 * (fscrypt_policy.version = 2).
+		 */
+		pr_warn_once("%s (pid %d) is setting less secure v0 encryption policy; recommend upgrading to v2.\n",
+			     current->comm, current->pid);
+	} else {
+		ret = fscrypt_compute_key_hash(inode, &policy, key_hash);
+		if (ret)
+			return ret;
+	}
+
 	ret = mnt_want_write_file(filp);
 	if (ret)
 		return ret;
@@ -114,10 +138,12 @@  int fscrypt_ioctl_set_policy(struct file *filp, const void __user *arg)
 			ret = -ENOTEMPTY;
 		else
 			ret = create_encryption_context_from_policy(inode,
-								    &policy);
+								    &policy,
+								    key_hash);
 	} else if (ret >= 0 && fscrypt_valid_context_format(&ctx, ret) &&
 		   is_encryption_context_consistent_with_policy(&ctx,
-								&policy)) {
+								&policy,
+								key_hash)) {
 		/* The file already uses the same encryption policy. */
 		ret = 0;
 	} else if (ret >= 0 || ret == -ERANGE) {
@@ -234,7 +260,11 @@  int fscrypt_has_permitted_context(struct inode *parent, struct inode *child)
 			(parent_ci->ci_data_mode == child_ci->ci_data_mode) &&
 			(parent_ci->ci_filename_mode ==
 			 child_ci->ci_filename_mode) &&
-			(parent_ci->ci_flags == child_ci->ci_flags);
+			(parent_ci->ci_flags == child_ci->ci_flags) &&
+			(parent_ci->ci_context_version == FSCRYPT_CONTEXT_V1 ||
+			 (memcmp(parent_ci->ci_master_key->mk_hash,
+				 child_ci->ci_master_key->mk_hash,
+				 FSCRYPT_KEY_HASH_SIZE) == 0));
 	}
 
 	res = cops->get_context(parent, &parent_ctx, sizeof(parent_ctx));
@@ -253,7 +283,10 @@  int fscrypt_has_permitted_context(struct inode *parent, struct inode *child)
 		 child_ctx.contents_encryption_mode) &&
 		(parent_ctx.filenames_encryption_mode ==
 		 child_ctx.filenames_encryption_mode) &&
-		(parent_ctx.flags == child_ctx.flags);
+		(parent_ctx.flags == child_ctx.flags) &&
+		(parent_ctx.version == FSCRYPT_CONTEXT_V1 ||
+		 (memcmp(parent_ctx.key_hash, child_ctx.key_hash,
+			 FSCRYPT_KEY_HASH_SIZE) == 0));
 }
 EXPORT_SYMBOL(fscrypt_has_permitted_context);
 
@@ -288,8 +321,10 @@  int fscrypt_inherit_context(struct inode *parent, struct inode *child,
 	memcpy(ctx.master_key_descriptor, ci->ci_master_key_descriptor,
 	       FS_KEY_DESCRIPTOR_SIZE);
 	get_random_bytes(ctx.nonce, FS_KEY_DERIVATION_NONCE_SIZE);
-	if (ctx.version != FSCRYPT_CONTEXT_V1)
-		memset(ctx.key_hash, 0, FSCRYPT_KEY_HASH_SIZE);
+	if (ctx.version != FSCRYPT_CONTEXT_V1) {
+		memcpy(ctx.key_hash, ci->ci_master_key->mk_hash,
+		       FSCRYPT_KEY_HASH_SIZE);
+	}
 
 	BUILD_BUG_ON(sizeof(ctx) != FSCRYPT_SET_CONTEXT_MAX_SIZE);
 	res = parent->i_sb->s_cop->set_context(child, &ctx,