Re: [PATCH v1.3 4/4] keys: add new key-type encrypted

[Mimi Zohar]

On Fri, 2010-11-12 at 19:45 +0000, David Howells wrote:
> Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx> wrote:
> 
> > Defines a new kernel key-type called 'encrypted'. Encrypted keys are
> 
> Many of the comments I made against patch #3 also apply here.  Use 'Define'
> rather than 'Defines' here for example.

Was expecting your comments. Already started making the changes ...

> > index 0000000..e2312e0
> > --- /dev/null
> > +++ b/include/keys/encrypted-type.h
> > @@ -0,0 +1,30 @@
> > +/* encrypted-type.h: encrypted-defined key type
> 
> Don't include the names of files in the files.
> 
> > +struct encrypted_key_payload {
> > +	struct rcu_head rcu;	/* RCU destructor */
> 
> That comment is not really needed.
> 
> > +	char *master_desc;	/* datablob: master key name */
> > +	char *datalen;		/* datablob: decrypted key length */
> > +	void *iv;		/* datablob: iv */
> > +	void *encrypted_data;	/* datablob: encrypted key */
> 
> But the variable name is 'encrypted_data'...
> 
> > +	unsigned short datablob_len;	/* length of datablob */
> > +	unsigned short decrypted_datalen;	/* decrypted data length */
> > +	char decrypted_data[0];	/* decrypted data +  datablob + hmac */
> 
> If any of these are binary data rather than character data, I'd use u8 rather
> than char.
> 
> > --- /dev/null
> > +++ b/security/keys/encrypted_defined.c
> > @@ -0,0 +1,816 @@
> > +/*
> > + * Copyright (C) 2010 IBM Corporation
> > + *
> > + * Author:
> > + * Mimi Zohar <zohar@xxxxxxxxxx>
> > + *
> > + * This program is free software; you can redistribute it and/or modify
> > + * it under the terms of the GNU General Public License as published by
> > + * the Free Software Foundation, version 2 of the License.
> > + *
> > + * File: encrypted_defined.c
> 
> Don't do that.
> 
> > + *
> > + * Defines a new kernel key-type called 'encrypted'. Encrypted keys
> > + * are kernel generated random numbers, which are encrypted/decrypted
> > + * using a 'master' key. The 'master' key can either be a trusted-key or
> > + * user-key type. Encrypted keys are created/encrypted/decrypted in the
> > + * kernel. Userspace ever only sees/stores encrypted blobs.
> > + *
> > + * keyctl add "encrypted" "name" "NEW master-key-name keylen" ring
> > + * keyctl add "encrypted" "name" "LOAD master-key-name keylen hex_blob" ring
> > + * keyctl update keyid "UPDATE master-key-name"
> 
> Merge with the documentation file in Documentation/ from patch 3 and stick a
> pointer to that file here.
> 
> > +static char hash_alg[] = "sha256";
> > +static char hmac_alg[] = "hmac(sha256)";
> 
> const.
> 
> > +static int hash_size = SHA256_DIGEST_SIZE;
> 
> ???  This is a numeric constant, but you're telling the compiler you want to
> store it in the .data section.  Is there a reason?
> 
> > +static char blkcipher_alg[] = "cbc(aes)";
> 
> const.
> 
> > +static int ivsize;
> > +static int blksize;
> > +static int MAX_DATA_SIZE = 4096;
> > +static int MIN_DATA_SIZE = 20;
> 
> Initialised numeric constants.
> 
> > +static int aes_get_sizes(int *ivsize, int *blksize)
> 
> It's only used in one place in the file, and it only sets global vars.  Why
> pass pointers to them?

agreed

> > +enum {
> > +	Opt_err = -1, Opt_new = 1, Opt_load,
> > +	Opt_update, Opt_NEW, Opt_LOAD, Opt_UPDATE
> > +};
> 
> Why skip 0?
> 
> > +static match_table_t key_tokens = {
> 
> const.
> 
> > +	{Opt_new, "new"},
> > +	{Opt_NEW, "NEW"},
> > +	{Opt_load, "load"},
> > +	{Opt_LOAD, "LOAD"},
> > +	{Opt_update, "update"},
> > +	{Opt_UPDATE, "UPDATE"},
> > +	{Opt_err, NULL}
> 
> Is case significant?  If not, you don't need duplicated constants.  Why do you
> care about the capitalisation here (and only in a restricted sense) when you
> didn't in the trusted keys patch?

No, was being consistent with trusted keys, and then uppercase was
removed there, but left here. Will remove.

> > +/* datablob_format - format as an ascii string, before copying to userspace */
> 
> I would split this over three lines.
> 
> > +static int datablob_format(char __user *buffer,
> > +			   struct encrypted_key_payload *epayload,
> > +			   int asciiblob_len)
> > +{
> > +	char *ascii_buf, *bufp;
> > +	char *iv = (char *)epayload->iv;
> 
> Unnecessary cast.  iv is void*.
> 
> > +	int ret = 0;
> > +	int len;
> > +	int i;
> > +
> > +	ascii_buf = kmalloc(asciiblob_len + 1, GFP_KERNEL);
> > +	if (!ascii_buf)
> > +		return -ENOMEM;
> > +
> > +	*(ascii_buf + asciiblob_len) = '\0';
> 
> That's what square brackets are for.

Yes, that would be clearer.

> > +	len = sprintf(ascii_buf, "%s %s ", epayload->master_desc,
> > +		      epayload->datalen);
> 
> datalen is not a string.

I know I already fixed this. Somehow it crept back in.

> > +static struct key *request_trusted_key(char *trusted_desc, void **master_key,
> > +				       unsigned int *master_keylen)
> > +{
> > +	struct trusted_key_payload *tpayload;
> > +	struct key *tkey;
> > +
> > +	tkey = request_key(&key_type_trusted, trusted_desc, NULL);
> > +	if (IS_ERR(tkey))
> > +		goto error;
> > +
> > +	tpayload = tkey->payload.data;
> > +	*master_key = tpayload->key;
> > +	*master_keylen = tpayload->key_len;
> 
> You can't do this.  You've defined an update method, so the pointer may change
> under you, and the pointee may be destroyed without warning.  You've been
> using RCU, so you should use that.  You need to use rcu_read_{,un}lock() and
> rcu_dereference() and you must be aware that the payload may be destroyed from
> the moment you drop the RCU read lock.

Thanks for explaining the locking necessary here. Update of an encrypted
key changes the name of the key used to encrypt the data. There are no
guarantees that the key will remain around. So I'm not concerned that
after we release the RCU lock, the key could disappear.

> You're returning a pointer to the key, so you perhaps don't really need to
> access the payload at this point.

Returning a pointer to the key was in order to do the key_put.  The
locking probably should be done in request_master_key.

> > +static struct key *request_user_key(char *master_desc, void **master_key,
> > +				    unsigned int *master_keylen)
> > +{
> > +	struct user_key_payload *upayload;
> > +	struct key *ukey;
> > +
> > +	ukey = request_key(&key_type_user, master_desc, NULL);
> > +	if (IS_ERR(ukey))
> > +		goto error;
> > +
> > +	upayload = ukey->payload.data;
> > +	*master_key = upayload->data;
> > +	*master_keylen = (unsigned int)upayload->datalen;
> 
> Ditto.
> 
> > +static int init_desc(struct hash_desc *desc, char *alg)
> > +{
> > +	int ret;
> > +
> > +	desc->tfm = crypto_alloc_hash(alg, 0, CRYPTO_ALG_ASYNC);
> 
> Use shash so that you don't have to use the SG interface.

Will look into it.

> > +static int calc_hmac(char *digest, char *key, int keylen,
> > +		     char *buf, size_t buflen)
> 
> Can key be const?  Should digest, key and buf be u8* or void* if they're
> binary data rather than string data?  This sort of thing should perhaps be
> percolated through all your functions.

Yes, will review the types used through out, adding const when possible.

> > +static int calc_hash(char *digest, void *buf, int buflen)
> 
> Can buf be const?
> 
> > +static int get_derived_key(char *derived_key, enum derived_key_type key_type,
> > +			   void *master_key, unsigned int master_keylen)
> 
> master_key should be const.  master_keylen should perhaps be size_t.
> 
> > +static struct key *request_master_key(struct encrypted_key_payload *epayload,
> > +				      void **master_key,
> > +				      unsigned int *master_keylen)
> > +{
> > +	struct key *mkey;
> > +
> > +	mkey = request_trusted_key(epayload->master_desc,
> > +				   master_key, master_keylen);
> > +	if (IS_ERR(mkey)) {
> > +		mkey = request_user_key(epayload->master_desc,
> > +					master_key, master_keylen);
> > +		if (IS_ERR(mkey)) {
> > +			pr_info("encrypted_key: trusted/user key %s not found",
> > +				epayload->master_desc);
> > +			return mkey;
> > +		}
> > +	}
> > +	dump_master_key(*master_key, *master_keylen);
> > +	return mkey;
> > +}
> 
> Why do you allow the master key to be supplied by a user-defined key rather
> than requiring a trusted-key unconditionally?

This is for systems without a TPM. The logic needs to exist, whether it
is here or in EVM.  By doing it here, a user could provide a passphrase
in the initramfs, which is used to decrypt the encrypted key.

> Note that the description of a user-defined key should prefixed to distinguish
> it from other random user-defined keys.
> 
> > +static int derived_key_encrypt(struct encrypted_key_payload *epayload,
> > +			       void *derived_key, unsigned int derived_keylen)
> 
> Should derived_key be const and derived_keylen be size_t?
> 
> > +static int datablob_hmac_append(struct encrypted_key_payload *epayload,
> > +				void *master_key, unsigned int master_keylen)
> 
> More const and size_t?  There are plenty more places where you should probably
> be using const or size_t.  I'm not going to list any further
> 
> > +static void encrypted_rcu_free(struct rcu_head *rcu)
> > +{
> > +	struct encrypted_key_payload *epayload;
> > +
> > +	epayload = container_of(rcu, struct encrypted_key_payload, rcu);
> > +	memset(epayload->decrypted_data, 0, epayload->decrypted_datalen);
> > +	kfree(epayload);
> 
> I presume you don't need to release the stuff pointed to by epayload before
> the memset because __ekey_init() makes the pointers point at offsets into the
> payload blob.

right

> > +static int __init init_encrypted(void)
> > +{
> > +	int ret;
> > +
> > +	ret = register_key_type(&key_type_encrypted);
> > +	if (ret < 0)
> > +		return ret;
> > +	ret = aes_get_sizes(&ivsize, &blksize);
> > +	return ret;
> 
> Merge those two lines into one.

ok

> > +static inline void dump_master_key(void *master_key, unsigned int master_keylen)
> > +{
> > +	print_hex_dump(KERN_ERR, "master key: ", DUMP_PREFIX_NONE, 32, 1,
> > +		       master_key, (size_t) master_keylen, 0);
> 
> You don't need to cast master_keylen like this.  The compiler will do that
> automatically.  In fact, master_key should be const and master_key_len should
> probably be size_t.
> 
> > +static inline void dump_decrypted_data(struct encrypted_key_payload
> > *epayload)
> 
> const.
> 
> > +static inline void dump_encrypted_data(struct encrypted_key_payload *epayload,
> > +				       unsigned int encrypted_datalen)
> 
> const and size_t.  And further instances thereof further down.
> 
> David

thanks,

Mimi

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