Subject: Re: [PATCH v3 1/7] integrity: Introduce struct evm_hmac_xattr

From: Mimi Zohar <[email-masked]>
Date: Fri, 28 Jul 2017 08:39:26 -0400

Hi Thiago,

On Thu, 2017-07-06 at 19:17 -0300, Thiago Jung Bauermann wrote:
> Even though struct evm_ima_xattr_data includes a fixed-size array to hold a
> SHA1 digest, most of the code ignores the array and uses the struct to mean
> "type indicator followed by data of unspecified size" and tracks the real
> size of what the struct represents in a separate length variable.
> 
> The only exception to that is the EVM code, which correctly uses the
> definition of struct evm_ima_xattr_data.

Right, the current EVM code converts the EVM signature to an HMAC the
first time the file is accessed.  So most of the code is based on the
HMAC.

> 
> This patch makes this explicit in the code by removing the length
> specification from the array in struct evm_ima_xattr_data. It also changes
> the name of the element from digest to data, since in most places the array
> doesn't hold a digest.
> 
> A separate struct evm_hmac_xattr is introduced, with the original
> definition of evm_ima_xattr_data to be used in the places that actually
> expect that definition.

The new structure name implies that the xattr can only be an HMAC.  By
moving the new structure to evm.h we also loose the connection that it
has to the evm_ima_xattr_type enumeration.

Instead, how about defining the new struct in terms of the modified
evm_ima_xattr_data struct?  Please leave the new structure in
integrity.h.

Mimi

> 
> Signed-off-by: Thiago Jung Bauermann <[email-masked]>

> ---
>  security/integrity/evm/evm.h          | 5 +++++
>  security/integrity/evm/evm_crypto.c   | 2 +-
>  security/integrity/evm/evm_main.c     | 8 ++++----
>  security/integrity/ima/ima_appraise.c | 7 ++++---
>  security/integrity/integrity.h        | 2 +-
>  5 files changed, 15 insertions(+), 9 deletions(-)
> 
> diff --git a/security/integrity/evm/evm.h b/security/integrity/evm/evm.h
> index f5f12727771a..e1081cf2f9c5 100644
> --- a/security/integrity/evm/evm.h
> +++ b/security/integrity/evm/evm.h
> @@ -24,6 +24,11 @@
>  #define EVM_INIT_HMAC	0x0001
>  #define EVM_INIT_X509	0x0002
> 
> +struct evm_hmac_xattr {
> +	u8 type;	/* Should be EVM_XATTR_HMAC. */
> +	u8 digest[SHA1_DIGEST_SIZE];
> +} __packed;
> +
>  extern int evm_initialized;
>  extern char *evm_hmac;
>  extern char *evm_hash;
> diff --git a/security/integrity/evm/evm_crypto.c b/security/integrity/evm/evm_crypto.c
> index d7f282d75cc1..08dde59f3128 100644
> --- a/security/integrity/evm/evm_crypto.c
> +++ b/security/integrity/evm/evm_crypto.c
> @@ -252,7 +252,7 @@ int evm_update_evmxattr(struct dentry *dentry, const char *xattr_name,
>  			const char *xattr_value, size_t xattr_value_len)
>  {
>  	struct inode *inode = d_backing_inode(dentry);
> -	struct evm_ima_xattr_data xattr_data;
> +	struct evm_hmac_xattr xattr_data;
>  	int rc = 0;
> 
>  	rc = evm_calc_hmac(dentry, xattr_name, xattr_value,
> diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> index 063d38aef64e..b7c1e11a915e 100644
> --- a/security/integrity/evm/evm_main.c
> +++ b/security/integrity/evm/evm_main.c
> @@ -116,7 +116,7 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
>  					     struct integrity_iint_cache *iint)
>  {
>  	struct evm_ima_xattr_data *xattr_data = NULL;
> -	struct evm_ima_xattr_data calc;
> +	struct evm_hmac_xattr calc;
>  	enum integrity_status evm_status = INTEGRITY_PASS;
>  	int rc, xattr_len;
> 
> @@ -147,7 +147,7 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
>  	/* check value type */
>  	switch (xattr_data->type) {
>  	case EVM_XATTR_HMAC:
> -		if (xattr_len != sizeof(struct evm_ima_xattr_data)) {
> +		if (xattr_len != sizeof(struct evm_hmac_xattr)) {
>  			evm_status = INTEGRITY_FAIL;
>  			goto out;
>  		}
> @@ -155,7 +155,7 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
>  				   xattr_value_len, calc.digest);
>  		if (rc)
>  			break;
> -		rc = crypto_memneq(xattr_data->digest, calc.digest,
> +		rc = crypto_memneq(xattr_data->data, calc.digest,
>  			    sizeof(calc.digest));
>  		if (rc)
>  			rc = -EINVAL;
> @@ -467,7 +467,7 @@ int evm_inode_init_security(struct inode *inode,
>  				 const struct xattr *lsm_xattr,
>  				 struct xattr *evm_xattr)
>  {
> -	struct evm_ima_xattr_data *xattr_data;
> +	struct evm_hmac_xattr *xattr_data;
>  	int rc;
> 
>  	if (!evm_initialized || !evm_protected_xattr(lsm_xattr->name))
> diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
> index 809ba70fbbbf..87d2b601cf8e 100644
> --- a/security/integrity/ima/ima_appraise.c
> +++ b/security/integrity/ima/ima_appraise.c
> @@ -156,7 +156,8 @@ enum hash_algo ima_get_hash_algo(struct evm_ima_xattr_data *xattr_value,
>  		return sig->hash_algo;
>  		break;
>  	case IMA_XATTR_DIGEST_NG:
> -		ret = xattr_value->digest[0];
> +		/* first byte contains algorithm id */
> +		ret = xattr_value->data[0];
>  		if (ret < HASH_ALGO__LAST)
>  			return ret;
>  		break;
> @@ -164,7 +165,7 @@ enum hash_algo ima_get_hash_algo(struct evm_ima_xattr_data *xattr_value,
>  		/* this is for backward compatibility */
>  		if (xattr_len == 21) {
>  			unsigned int zero = 0;
> -			if (!memcmp(&xattr_value->digest[16], &zero, 4))
> +			if (!memcmp(&xattr_value->data[16], &zero, 4))
>  				return HASH_ALGO_MD5;
>  			else
>  				return HASH_ALGO_SHA1;
> @@ -253,7 +254,7 @@ int ima_appraise_measurement(enum ima_hooks func,
>  			/* xattr length may be longer. md5 hash in previous
>  			   version occupied 20 bytes in xattr, instead of 16
>  			 */
> -			rc = memcmp(&xattr_value->digest[hash_start],
> +			rc = memcmp(&xattr_value->data[hash_start],
>  				    iint->ima_hash->digest,
>  				    iint->ima_hash->length);
>  		else
> diff --git a/security/integrity/integrity.h b/security/integrity/integrity.h
> index a53e7e4ab06c..874211aba6e9 100644
> --- a/security/integrity/integrity.h
> +++ b/security/integrity/integrity.h
> @@ -63,7 +63,7 @@ enum evm_ima_xattr_type {
> 
>  struct evm_ima_xattr_data {
>  	u8 type;
> -	u8 digest[SHA1_DIGEST_SIZE];
> +	u8 data[];
>  } __packed;
> 
>  #define IMA_MAX_DIGEST_SIZE	64