Re: [PATCH v3 1/2] selinux: add brief info to policydb

[Casey Schaufler]

On 5/11/2017 5:59 AM, Sebastien Buisson wrote:
> Add policybrief field to struct policydb. It holds a brief info
> of the policydb, in the following form:
> <0 or 1 for enforce>:<0 or 1 for checkreqprot>:<hashalg>=<checksum>
> Policy brief is computed every time the policy is loaded, and when
> enforce or checkreqprot are changed.
>
> Add security_policy_brief hook to give access to policy brief to
> the rest of the kernel. Lustre client makes use of this information
> to detect changes to the policy, and forward it to Lustre servers.
> Depending on how the policy is enforced on Lustre client side,
> Lustre servers can refuse connection.
>
> Signed-off-by: Sebastien Buisson <sbuisson@xxxxxxx>
> ---
>  include/linux/lsm_hooks.h           | 16 ++++++++
>  include/linux/security.h            |  7 ++++
>  security/security.c                 |  6 +++
>  security/selinux/hooks.c            |  7 ++++
>  security/selinux/include/security.h |  2 +
>  security/selinux/selinuxfs.c        |  2 +
>  security/selinux/ss/policydb.c      | 76 +++++++++++++++++++++++++++++++++++++
>  security/selinux/ss/policydb.h      |  2 +
>  security/selinux/ss/services.c      | 62 ++++++++++++++++++++++++++++++
>  9 files changed, 180 insertions(+)
>
> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> index 080f34e..9cac282 100644
> --- a/include/linux/lsm_hooks.h
> +++ b/include/linux/lsm_hooks.h
> @@ -1336,6 +1336,20 @@
>   *	@inode we wish to get the security context of.
>   *	@ctx is a pointer in which to place the allocated security context.
>   *	@ctxlen points to the place to put the length of @ctx.
> + *
> + * Security hooks for policy brief
> + *
> + * @policy_brief:
> + *
> + *	Returns a string containing a brief info of the policydb, in the
> + *	following form:
> + *	<0 or 1 for enforce>:<0 or 1 for checkreqprot>:<hashalg>=<checksum>

This sure looks like SELinux specific information. If the Spiffy
security module has multiple values for enforcement (e.g. off,
soft and hard) this interface definition does not work. What is a
"checkreqprot", and what is it for?

I expect that you have no interest (or incentive) in supporting
security modules other than SELinux, and that's OK. What's I'm
after is an interface that another security module could use if
someone where interested (or inspired) to do so.

Rather than a string with predefined positional values (something
I was taught not to do when 1 MIPS and 1 MEG was a big computer)
you might use "enforce=<value>:checkreqprot=<value>:hashalg=<checksum>"
for SELinux and define @policy_brief as

	A string containing colon separated name and value pairs
	that will be parsed and interpreted by the security module
	or modules.

You already have it right for the "hashalg" field. If you want to
be really forward looking you could use names field names that
identify the security module that uses them, such as

	"selinux/enforce=1:selinux/checkreqprot=0:selinux/MD5=8675309"

Note that I put "selinux/" onto the MD5 as well. You may have
multiple modules that use this interface on a system at the same
time in the not to distant future:

	selinux/enforce=1:selinux/checkreqprot=0:
	selinux/MD5=8675309:spiffy/enforce=soft:
	spiffy/updatefreq=32544:spiffy/SHA256=84521

If you deal with this now it won't be a major headache to deal with
later.

> + *
> + *	@brief: pointer to buffer holding brief
> + *	@len: in: brief buffer length if no alloc, out: brief string len
> + *	@alloc: whether to allocate buffer for brief or not
> + *	On success 0 is returned , or negative value on error.
> + *
>   * This is the main security structure.
>   */
>  
> @@ -1568,6 +1582,7 @@
>  	int (*inode_setsecctx)(struct dentry *dentry, void *ctx, u32 ctxlen);
>  	int (*inode_getsecctx)(struct inode *inode, void **ctx, u32 *ctxlen);
>  
> +	int (*policy_brief)(char **brief, size_t *len, bool alloc);
>  #ifdef CONFIG_SECURITY_NETWORK
>  	int (*unix_stream_connect)(struct sock *sock, struct sock *other,
>  					struct sock *newsk);
> @@ -1813,6 +1828,7 @@ struct security_hook_heads {
>  	struct list_head inode_notifysecctx;
>  	struct list_head inode_setsecctx;
>  	struct list_head inode_getsecctx;
> +	struct list_head policy_brief;
>  #ifdef CONFIG_SECURITY_NETWORK
>  	struct list_head unix_stream_connect;
>  	struct list_head unix_may_send;
> diff --git a/include/linux/security.h b/include/linux/security.h
> index af675b5..3b72053 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -377,6 +377,8 @@ int security_sem_semop(struct sem_array *sma, struct sembuf *sops,
>  int security_inode_notifysecctx(struct inode *inode, void *ctx, u32 ctxlen);
>  int security_inode_setsecctx(struct dentry *dentry, void *ctx, u32 ctxlen);
>  int security_inode_getsecctx(struct inode *inode, void **ctx, u32 *ctxlen);
> +
> +int security_policy_brief(char **brief, size_t *len, bool alloc);
>  #else /* CONFIG_SECURITY */
>  struct security_mnt_opts {
>  };
> @@ -1166,6 +1168,11 @@ static inline int security_inode_getsecctx(struct inode *inode, void **ctx, u32
>  {
>  	return -EOPNOTSUPP;
>  }
> +
> +static inline int security_policy_brief(char **brief, size_t *len, bool alloc)
> +{
> +	return -EOPNOTSUPP;
> +}
>  #endif	/* CONFIG_SECURITY */
>  
>  #ifdef CONFIG_SECURITY_NETWORK
> diff --git a/security/security.c b/security/security.c
> index b9fea39..954b391 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -1285,6 +1285,12 @@ int security_inode_getsecctx(struct inode *inode, void **ctx, u32 *ctxlen)
>  }
>  EXPORT_SYMBOL(security_inode_getsecctx);
>  
> +int security_policy_brief(char **brief, size_t *len, bool alloc)
> +{
> +	return call_int_hook(policy_brief, -EOPNOTSUPP, brief, len, alloc);
> +}
> +EXPORT_SYMBOL(security_policy_brief);
> +
>  #ifdef CONFIG_SECURITY_NETWORK
>  
>  int security_unix_stream_connect(struct sock *sock, struct sock *other, struct sock *newsk)
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index e67a526..da245e8 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -6063,6 +6063,11 @@ static int selinux_inode_getsecctx(struct inode *inode, void **ctx, u32 *ctxlen)
>  	*ctxlen = len;
>  	return 0;
>  }
> +
> +static int selinux_policy_brief(char **brief, size_t *len, bool alloc)
> +{
> +	return security_policydb_brief(brief, len, alloc);
> +}
>  #ifdef CONFIG_KEYS
>  
>  static int selinux_key_alloc(struct key *k, const struct cred *cred,
> @@ -6277,6 +6282,8 @@ static int selinux_key_getsecurity(struct key *key, char **_buffer)
>  	LSM_HOOK_INIT(inode_setsecctx, selinux_inode_setsecctx),
>  	LSM_HOOK_INIT(inode_getsecctx, selinux_inode_getsecctx),
>  
> +	LSM_HOOK_INIT(policy_brief, selinux_policy_brief),
> +
>  	LSM_HOOK_INIT(unix_stream_connect, selinux_socket_unix_stream_connect),
>  	LSM_HOOK_INIT(unix_may_send, selinux_socket_unix_may_send),
>  
> diff --git a/security/selinux/include/security.h b/security/selinux/include/security.h
> index f979c35..a0d4d7d 100644
> --- a/security/selinux/include/security.h
> +++ b/security/selinux/include/security.h
> @@ -97,6 +97,8 @@ enum {
>  int security_load_policy(void *data, size_t len);
>  int security_read_policy(void **data, size_t *len);
>  size_t security_policydb_len(void);
> +int security_policydb_brief(char **brief, size_t *len, bool alloc);
> +void security_policydb_update_info(u32 requested);
>  
>  int security_policycap_supported(unsigned int req_cap);
>  
> diff --git a/security/selinux/selinuxfs.c b/security/selinux/selinuxfs.c
> index 50062e7..8c9f5b7 100644
> --- a/security/selinux/selinuxfs.c
> +++ b/security/selinux/selinuxfs.c
> @@ -159,6 +159,7 @@ static ssize_t sel_write_enforce(struct file *file, const char __user *buf,
>  			from_kuid(&init_user_ns, audit_get_loginuid(current)),
>  			audit_get_sessionid(current));
>  		selinux_enforcing = new_value;
> +		security_policydb_update_info(SECURITY__SETENFORCE);
>  		if (selinux_enforcing)
>  			avc_ss_reset(0);
>  		selnl_notify_setenforce(selinux_enforcing);
> @@ -621,6 +622,7 @@ static ssize_t sel_write_checkreqprot(struct file *file, const char __user *buf,
>  		goto out;
>  
>  	selinux_checkreqprot = new_value ? 1 : 0;
> +	security_policydb_update_info(SECURITY__SETCHECKREQPROT);
>  	length = count;
>  out:
>  	kfree(page);
> diff --git a/security/selinux/ss/policydb.c b/security/selinux/ss/policydb.c
> index 0080122..58e73f5 100644
> --- a/security/selinux/ss/policydb.c
> +++ b/security/selinux/ss/policydb.c
> @@ -32,11 +32,13 @@
>  #include <linux/errno.h>
>  #include <linux/audit.h>
>  #include <linux/flex_array.h>
> +#include <crypto/hash.h>
>  #include "security.h"
>  
>  #include "policydb.h"
>  #include "conditional.h"
>  #include "mls.h"
> +#include "objsec.h"
>  #include "services.h"
>  
>  #define _DEBUG_HASHES
> @@ -879,6 +881,8 @@ void policydb_destroy(struct policydb *p)
>  	ebitmap_destroy(&p->filename_trans_ttypes);
>  	ebitmap_destroy(&p->policycaps);
>  	ebitmap_destroy(&p->permissive_map);
> +
> +	kfree(p->policybrief);
>  }
>  
>  /*
> @@ -2220,6 +2224,73 @@ static int ocontext_read(struct policydb *p, struct policydb_compat_info *info,
>  }
>  
>  /*
> + * Compute summary of a policy database binary representation file,
> + * and store it into a policy database structure.
> + */
> +static int policydb_brief(struct policydb *policydb, void *ptr)
> +{
> +	struct policy_file *fp = ptr;
> +	struct crypto_shash *tfm;
> +	char hashalg[] = "sha256";
> +	size_t hashsize;
> +	u8 *hashval;
> +	int idx;
> +	unsigned char *p;
> +
> +	if (policydb->policybrief)
> +		return -EINVAL;
> +
> +	tfm = crypto_alloc_shash(hashalg, 0, 0);
> +	if (IS_ERR(tfm)) {
> +		printk(KERN_ERR "Failed to alloc crypto hash %s\n", hashalg);
> +		return PTR_ERR(tfm);
> +	}
> +
> +	hashsize = crypto_shash_digestsize(tfm);
> +	hashval = kmalloc(hashsize, GFP_KERNEL);
> +	if (hashval == NULL) {
> +		crypto_free_shash(tfm);
> +		return -ENOMEM;
> +	}
> +
> +	{
> +		int rc;
> +
> +		SHASH_DESC_ON_STACK(desc, tfm);
> +		desc->tfm = tfm;
> +		desc->flags = 0;
> +		rc = crypto_shash_digest(desc, fp->data, fp->len, hashval);
> +		crypto_free_shash(tfm);
> +		if (rc) {
> +			printk(KERN_ERR "Failed crypto_shash_digest: %d\n", rc);
> +			kfree(hashval);
> +			return rc;
> +		}
> +	}
> +
> +	/* policy brief is in the form:
> +	 * <0 or 1 for enforce>:<0 or 1 for checkreqprot>:<hashalg>=<checksum>
> +	 */
> +	policydb->policybrief = kzalloc(5 + strlen(hashalg) + 2*hashsize + 1,
> +					GFP_KERNEL);
> +	if (policydb->policybrief == NULL) {
> +		kfree(hashval);
> +		return -ENOMEM;
> +	}
> +
> +	sprintf(policydb->policybrief, "%d:%d:%s=",
> +		selinux_enforcing, selinux_checkreqprot, hashalg);
> +	p = policydb->policybrief + strlen(policydb->policybrief);
> +	for (idx = 0; idx < hashsize; idx++) {
> +		snprintf(p, 3, "%02x", hashval[idx]);
> +		p += 2;
> +	}
> +	kfree(hashval);
> +
> +	return 0;
> +}
> +
> +/*
>   * Read the configuration data from a policy database binary
>   * representation file into a policy database structure.
>   */
> @@ -2238,6 +2309,11 @@ int policydb_read(struct policydb *p, void *fp)
>  	if (rc)
>  		return rc;
>  
> +	/* Compute summary of policy, and store it in policydb */
> +	rc = policydb_brief(p, fp);
> +	if (rc)
> +		goto bad;
> +
>  	/* Read the magic number and string length. */
>  	rc = next_entry(buf, fp, sizeof(u32) * 2);
>  	if (rc)
> diff --git a/security/selinux/ss/policydb.h b/security/selinux/ss/policydb.h
> index 725d594..31689d2f 100644
> --- a/security/selinux/ss/policydb.h
> +++ b/security/selinux/ss/policydb.h
> @@ -293,6 +293,8 @@ struct policydb {
>  	size_t len;
>  
>  	unsigned int policyvers;
> +	/* summary computed on the policy */
> +	unsigned char *policybrief;
>  
>  	unsigned int reject_unknown : 1;
>  	unsigned int allow_unknown : 1;
> diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
> index 60d9b02..3bbe649 100644
> --- a/security/selinux/ss/services.c
> +++ b/security/selinux/ss/services.c
> @@ -2170,6 +2170,68 @@ size_t security_policydb_len(void)
>  }
>  
>  /**
> + * security_policydb_brief - Get policydb brief
> + * @brief: pointer to buffer holding brief
> + * @len: in: brief buffer length if no alloc, out: brief string len
> + * @alloc: whether to allocate buffer for brief or not
> + *
> + * On success 0 is returned , or negative value on error.
> + **/
> +int security_policydb_brief(char **brief, size_t *len, bool alloc)
> +{
> +	size_t policybrief_len;
> +
> +	if (!ss_initialized || brief == NULL)
> +		return -EINVAL;
> +
> +	read_lock(&policy_rwlock);
> +	policybrief_len = strlen(policydb.policybrief);
> +	read_unlock(&policy_rwlock);
> +
> +	if (alloc)
> +		/* *brief must be kfreed by caller in this case */
> +		*brief = kzalloc(policybrief_len + 1, GFP_KERNEL);
> +	else
> +		/*
> +		 * if !alloc, caller must pass a buffer that
> +		 * can hold policybrief_len+1 chars
> +		 */
> +		if (*len < policybrief_len + 1) {
> +			/* put in *len the string size we need to write */
> +			*len = policybrief_len;
> +			return -ENAMETOOLONG;
> +		}
> +
> +	if (*brief == NULL)
> +		return -ENOMEM;
> +
> +	read_lock(&policy_rwlock);
> +	*len = strlen(policydb.policybrief);
> +	strncpy(*brief, policydb.policybrief, *len);
> +	read_unlock(&policy_rwlock);
> +
> +	return 0;
> +}
> +
> +void security_policydb_update_info(u32 requested)
> +{
> +	/* policy brief is in the form:
> +	 * <0 or 1 for enforce>:<0 or 1 for checkreqprot>:<hashalg>=<checksum>
> +	 */
> +
> +	if (!ss_initialized)
> +		return;
> +
> +	/* update global policydb, needs write lock */
> +	write_lock_irq(&policy_rwlock);
> +	if (requested == SECURITY__SETENFORCE)
> +		policydb.policybrief[0] = '0' + selinux_enforcing;
> +	else if (requested == SECURITY__SETCHECKREQPROT)
> +		policydb.policybrief[2] = '0' + selinux_checkreqprot;
> +	write_unlock_irq(&policy_rwlock);
> +}
> +
> +/**
>   * security_port_sid - Obtain the SID for a port.
>   * @protocol: protocol number
>   * @port: port number