Commit 3a3b7ce9 authored by David Howells's avatar David Howells Committed by James Morris

CRED: Allow kernel services to override LSM settings for task actions

Allow kernel services to override LSM settings appropriate to the actions
performed by a task by duplicating a set of credentials, modifying it and then
using task_struct::cred to point to it when performing operations on behalf of
a task.

This is used, for example, by CacheFiles which has to transparently access the
cache on behalf of a process that thinks it is doing, say, NFS accesses with a
potentially inappropriate (with respect to accessing the cache) set of
credentials.

This patch provides two LSM hooks for modifying a task security record:

 (*) security_kernel_act_as() which allows modification of the security datum
     with which a task acts on other objects (most notably files).

 (*) security_kernel_create_files_as() which allows modification of the
     security datum that is used to initialise the security data on a file that
     a task creates.

The patch also provides four new credentials handling functions, which wrap the
LSM functions:

 (1) prepare_kernel_cred()

     Prepare a set of credentials for a kernel service to use, based either on
     a daemon's credentials or on init_cred.  All the keyrings are cleared.

 (2) set_security_override()

     Set the LSM security ID in a set of credentials to a specific security
     context, assuming permission from the LSM policy.

 (3) set_security_override_from_ctx()

     As (2), but takes the security context as a string.

 (4) set_create_files_as()

     Set the file creation LSM security ID in a set of credentials to be the
     same as that on a particular inode.

Signed-off-by: Casey Schaufler <casey@schaufler-ca.com> [Smack changes]
Signed-off-by: default avatarDavid Howells <dhowells@redhat.com>
Signed-off-by: default avatarJames Morris <jmorris@namei.org>
parent 1bfdc75a
......@@ -18,6 +18,7 @@
struct user_struct;
struct cred;
struct inode;
/*
* COW Supplementary groups list
......@@ -148,6 +149,11 @@ extern int commit_creds(struct cred *);
extern void abort_creds(struct cred *);
extern const struct cred *override_creds(const struct cred *);
extern void revert_creds(const struct cred *);
extern struct cred *prepare_kernel_cred(struct task_struct *);
extern int change_create_files_as(struct cred *, struct inode *);
extern int set_security_override(struct cred *, u32);
extern int set_security_override_from_ctx(struct cred *, const char *);
extern int set_create_files_as(struct cred *, struct inode *);
extern void __init cred_init(void);
/**
......
......@@ -587,6 +587,19 @@ static inline void security_free_mnt_opts(struct security_mnt_opts *opts)
* @new points to the new credentials.
* @old points to the original credentials.
* Install a new set of credentials.
* @kernel_act_as:
* Set the credentials for a kernel service to act as (subjective context).
* @new points to the credentials to be modified.
* @secid specifies the security ID to be set
* The current task must be the one that nominated @secid.
* Return 0 if successful.
* @kernel_create_files_as:
* Set the file creation context in a set of credentials to be the same as
* the objective context of the specified inode.
* @new points to the credentials to be modified.
* @inode points to the inode to use as a reference.
* The current task must be the one that nominated @inode.
* Return 0 if successful.
* @task_setuid:
* Check permission before setting one or more of the user identity
* attributes of the current process. The @flags parameter indicates
......@@ -1381,6 +1394,8 @@ struct security_operations {
int (*cred_prepare)(struct cred *new, const struct cred *old,
gfp_t gfp);
void (*cred_commit)(struct cred *new, const struct cred *old);
int (*kernel_act_as)(struct cred *new, u32 secid);
int (*kernel_create_files_as)(struct cred *new, struct inode *inode);
int (*task_setuid) (uid_t id0, uid_t id1, uid_t id2, int flags);
int (*task_fix_setuid) (struct cred *new, const struct cred *old,
int flags);
......@@ -1632,6 +1647,8 @@ int security_task_create(unsigned long clone_flags);
void security_cred_free(struct cred *cred);
int security_prepare_creds(struct cred *new, const struct cred *old, gfp_t gfp);
void security_commit_creds(struct cred *new, const struct cred *old);
int security_kernel_act_as(struct cred *new, u32 secid);
int security_kernel_create_files_as(struct cred *new, struct inode *inode);
int security_task_setuid(uid_t id0, uid_t id1, uid_t id2, int flags);
int security_task_fix_setuid(struct cred *new, const struct cred *old,
int flags);
......@@ -2151,6 +2168,17 @@ static inline void security_commit_creds(struct cred *new,
{
}
static inline int security_kernel_act_as(struct cred *cred, u32 secid)
{
return 0;
}
static inline int security_kernel_create_files_as(struct cred *cred,
struct inode *inode)
{
return 0;
}
static inline int security_task_setuid(uid_t id0, uid_t id1, uid_t id2,
int flags)
{
......
......@@ -462,3 +462,116 @@ void __init cred_init(void)
cred_jar = kmem_cache_create("cred_jar", sizeof(struct cred),
0, SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL);
}
/**
* prepare_kernel_cred - Prepare a set of credentials for a kernel service
* @daemon: A userspace daemon to be used as a reference
*
* Prepare a set of credentials for a kernel service. This can then be used to
* override a task's own credentials so that work can be done on behalf of that
* task that requires a different subjective context.
*
* @daemon is used to provide a base for the security record, but can be NULL.
* If @daemon is supplied, then the security data will be derived from that;
* otherwise they'll be set to 0 and no groups, full capabilities and no keys.
*
* The caller may change these controls afterwards if desired.
*
* Returns the new credentials or NULL if out of memory.
*
* Does not take, and does not return holding current->cred_replace_mutex.
*/
struct cred *prepare_kernel_cred(struct task_struct *daemon)
{
const struct cred *old;
struct cred *new;
new = kmem_cache_alloc(cred_jar, GFP_KERNEL);
if (!new)
return NULL;
if (daemon)
old = get_task_cred(daemon);
else
old = get_cred(&init_cred);
get_uid(new->user);
get_group_info(new->group_info);
#ifdef CONFIG_KEYS
atomic_inc(&init_tgcred.usage);
new->tgcred = &init_tgcred;
new->request_key_auth = NULL;
new->thread_keyring = NULL;
new->jit_keyring = KEY_REQKEY_DEFL_THREAD_KEYRING;
#endif
#ifdef CONFIG_SECURITY
new->security = NULL;
#endif
if (security_prepare_creds(new, old, GFP_KERNEL) < 0)
goto error;
atomic_set(&new->usage, 1);
put_cred(old);
return new;
error:
put_cred(new);
return NULL;
}
EXPORT_SYMBOL(prepare_kernel_cred);
/**
* set_security_override - Set the security ID in a set of credentials
* @new: The credentials to alter
* @secid: The LSM security ID to set
*
* Set the LSM security ID in a set of credentials so that the subjective
* security is overridden when an alternative set of credentials is used.
*/
int set_security_override(struct cred *new, u32 secid)
{
return security_kernel_act_as(new, secid);
}
EXPORT_SYMBOL(set_security_override);
/**
* set_security_override_from_ctx - Set the security ID in a set of credentials
* @new: The credentials to alter
* @secctx: The LSM security context to generate the security ID from.
*
* Set the LSM security ID in a set of credentials so that the subjective
* security is overridden when an alternative set of credentials is used. The
* security ID is specified in string form as a security context to be
* interpreted by the LSM.
*/
int set_security_override_from_ctx(struct cred *new, const char *secctx)
{
u32 secid;
int ret;
ret = security_secctx_to_secid(secctx, strlen(secctx), &secid);
if (ret < 0)
return ret;
return set_security_override(new, secid);
}
EXPORT_SYMBOL(set_security_override_from_ctx);
/**
* set_create_files_as - Set the LSM file create context in a set of credentials
* @new: The credentials to alter
* @inode: The inode to take the context from
*
* Change the LSM file creation context in a set of credentials to be the same
* as the object context of the specified inode, so that the new inodes have
* the same MAC context as that inode.
*/
int set_create_files_as(struct cred *new, struct inode *inode)
{
new->fsuid = inode->i_uid;
new->fsgid = inode->i_gid;
return security_kernel_create_files_as(new, inode);
}
EXPORT_SYMBOL(set_create_files_as);
......@@ -348,6 +348,16 @@ static void cap_cred_commit(struct cred *new, const struct cred *old)
{
}
static int cap_kernel_act_as(struct cred *new, u32 secid)
{
return 0;
}
static int cap_kernel_create_files_as(struct cred *new, struct inode *inode)
{
return 0;
}
static int cap_task_setuid(uid_t id0, uid_t id1, uid_t id2, int flags)
{
return 0;
......@@ -889,6 +899,8 @@ void security_fixup_ops(struct security_operations *ops)
set_to_cap_if_null(ops, cred_free);
set_to_cap_if_null(ops, cred_prepare);
set_to_cap_if_null(ops, cred_commit);
set_to_cap_if_null(ops, kernel_act_as);
set_to_cap_if_null(ops, kernel_create_files_as);
set_to_cap_if_null(ops, task_setuid);
set_to_cap_if_null(ops, task_fix_setuid);
set_to_cap_if_null(ops, task_setgid);
......
......@@ -616,6 +616,16 @@ void security_commit_creds(struct cred *new, const struct cred *old)
return security_ops->cred_commit(new, old);
}
int security_kernel_act_as(struct cred *new, u32 secid)
{
return security_ops->kernel_act_as(new, secid);
}
int security_kernel_create_files_as(struct cred *new, struct inode *inode)
{
return security_ops->kernel_create_files_as(new, inode);
}
int security_task_setuid(uid_t id0, uid_t id1, uid_t id2, int flags)
{
return security_ops->task_setuid(id0, id1, id2, flags);
......
......@@ -3277,6 +3277,50 @@ static void selinux_cred_commit(struct cred *new, const struct cred *old)
secondary_ops->cred_commit(new, old);
}
/*
* set the security data for a kernel service
* - all the creation contexts are set to unlabelled
*/
static int selinux_kernel_act_as(struct cred *new, u32 secid)
{
struct task_security_struct *tsec = new->security;
u32 sid = current_sid();
int ret;
ret = avc_has_perm(sid, secid,
SECCLASS_KERNEL_SERVICE,
KERNEL_SERVICE__USE_AS_OVERRIDE,
NULL);
if (ret == 0) {
tsec->sid = secid;
tsec->create_sid = 0;
tsec->keycreate_sid = 0;
tsec->sockcreate_sid = 0;
}
return ret;
}
/*
* set the file creation context in a security record to the same as the
* objective context of the specified inode
*/
static int selinux_kernel_create_files_as(struct cred *new, struct inode *inode)
{
struct inode_security_struct *isec = inode->i_security;
struct task_security_struct *tsec = new->security;
u32 sid = current_sid();
int ret;
ret = avc_has_perm(sid, isec->sid,
SECCLASS_KERNEL_SERVICE,
KERNEL_SERVICE__CREATE_FILES_AS,
NULL);
if (ret == 0)
tsec->create_sid = isec->sid;
return 0;
}
static int selinux_task_setuid(uid_t id0, uid_t id1, uid_t id2, int flags)
{
/* Since setuid only affects the current process, and
......@@ -5593,6 +5637,8 @@ static struct security_operations selinux_ops = {
.cred_free = selinux_cred_free,
.cred_prepare = selinux_cred_prepare,
.cred_commit = selinux_cred_commit,
.kernel_act_as = selinux_kernel_act_as,
.kernel_create_files_as = selinux_kernel_create_files_as,
.task_setuid = selinux_task_setuid,
.task_fix_setuid = selinux_task_fix_setuid,
.task_setgid = selinux_task_setgid,
......
......@@ -1011,6 +1011,41 @@ static void smack_cred_commit(struct cred *new, const struct cred *old)
{
}
/**
* smack_kernel_act_as - Set the subjective context in a set of credentials
* @new points to the set of credentials to be modified.
* @secid specifies the security ID to be set
*
* Set the security data for a kernel service.
*/
static int smack_kernel_act_as(struct cred *new, u32 secid)
{
char *smack = smack_from_secid(secid);
if (smack == NULL)
return -EINVAL;
new->security = smack;
return 0;
}
/**
* smack_kernel_create_files_as - Set the file creation label in a set of creds
* @new points to the set of credentials to be modified
* @inode points to the inode to use as a reference
*
* Set the file creation context in a set of credentials to the same
* as the objective context of the specified inode
*/
static int smack_kernel_create_files_as(struct cred *new,
struct inode *inode)
{
struct inode_smack *isp = inode->i_security;
new->security = isp->smk_inode;
return 0;
}
/**
* smack_task_setpgid - Smack check on setting pgid
* @p: the task object
......@@ -2641,6 +2676,8 @@ struct security_operations smack_ops = {
.cred_free = smack_cred_free,
.cred_prepare = smack_cred_prepare,
.cred_commit = smack_cred_commit,
.kernel_act_as = smack_kernel_act_as,
.kernel_create_files_as = smack_kernel_create_files_as,
.task_fix_setuid = cap_task_fix_setuid,
.task_setpgid = smack_task_setpgid,
.task_getpgid = smack_task_getpgid,
......
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