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: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>

2 files changed, 34 insertions, 0 deletions

diff --git a/include/linux/cred.h b/include/linux/cred.h
index 55a9c995d69..26c1ab17994 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -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);
 
 /**
diff --git a/include/linux/security.h b/include/linux/security.h
index 56a0eed6567..59a11e19b61 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -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)
 {