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>

1 files changed, 12 insertions, 0 deletions

diff --git a/security/capability.c b/security/capability.c
index 185804f99ad..b9e391425e6 100644
--- a/security/capability.c
+++ b/security/capability.c
@@ -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);