lsm: separate security_task_getsecid() into subjective and objective variants

Of the three LSMs that implement the security_task_getsecid() LSM
hook, all three LSMs provide the task's objective security
credentials.  This turns out to be unfortunate as most of the hook's
callers seem to expect the task's subjective credentials, although
a small handful of callers do correctly expect the objective
credentials.

This patch is the first step towards fixing the problem: it splits
the existing security_task_getsecid() hook into two variants, one
for the subjective creds, one for the objective creds.

  void security_task_getsecid_subj(struct task_struct *p,
				   u32 *secid);
  void security_task_getsecid_obj(struct task_struct *p,
				  u32 *secid);

While this patch does fix all of the callers to use the correct
variant, in order to keep this patch focused on the callers and to
ease review, the LSMs continue to use the same implementation for
both hooks.  The net effect is that this patch should not change
the behavior of the kernel in any way, it will be up to the latter
LSM specific patches in this series to change the hook
implementations and return the correct credentials.

Acked-by: Mimi Zohar <[email protected]> (IMA)
Acked-by: Casey Schaufler <[email protected]>
Reviewed-by: Richard Guy Briggs <[email protected]>
Signed-off-by: Paul Moore <[email protected]>

Author: pcmoore

drivers/android/binder.c

@@ -2700,7 +2700,16 @@ static void binder_transaction(struct binder_proc *proc,
 		u32 secid;
 		size_t added_size;
 
-		security_task_getsecid(proc->tsk, &secid);
+		/*
+		 * Arguably this should be the task's subjective LSM secid but
+		 * we can't reliably access the subjective creds of a task
+		 * other than our own so we must use the objective creds, which
+		 * are safe to access.  The downside is that if a task is
+		 * temporarily overriding it's creds it will not be reflected
+		 * here; however, it isn't clear that binder would handle that
+		 * case well anyway.
+		 */
+		security_task_getsecid_obj(proc->tsk, &secid);
 		ret = security_secid_to_secctx(secid, &secctx, &secctx_sz);
 		if (ret) {
 			return_error = BR_FAILED_REPLY;

include/linux/cred.h

@@ -140,7 +140,7 @@ struct cred {
 	struct key	*request_key_auth; /* assumed request_key authority */
 #endif
 #ifdef CONFIG_SECURITY
-	void		*security;	/* subjective LSM security */
+	void		*security;	/* LSM security */
 #endif
 	struct user_struct *user;	/* real user ID subscription */
 	struct user_namespace *user_ns; /* user_ns the caps and keyrings are relative to. */

include/linux/lsm_hook_defs.h

@@ -204,7 +204,10 @@ LSM_HOOK(int, 0, task_fix_setgid, struct cred *new, const struct cred * old,
 LSM_HOOK(int, 0, task_setpgid, struct task_struct *p, pid_t pgid)
 LSM_HOOK(int, 0, task_getpgid, struct task_struct *p)
 LSM_HOOK(int, 0, task_getsid, struct task_struct *p)
-LSM_HOOK(void, LSM_RET_VOID, task_getsecid, struct task_struct *p, u32 *secid)
+LSM_HOOK(void, LSM_RET_VOID, task_getsecid_subj,
+	 struct task_struct *p, u32 *secid)
+LSM_HOOK(void, LSM_RET_VOID, task_getsecid_obj,
+	 struct task_struct *p, u32 *secid)
 LSM_HOOK(int, 0, task_setnice, struct task_struct *p, int nice)
 LSM_HOOK(int, 0, task_setioprio, struct task_struct *p, int ioprio)
 LSM_HOOK(int, 0, task_getioprio, struct task_struct *p)

include/linux/lsm_hooks.h

@@ -713,9 +713,15 @@
  *	@p.
  *	@p contains the task_struct for the process.
  *	Return 0 if permission is granted.
- * @task_getsecid:
- *	Retrieve the security identifier of the process @p.
- *	@p contains the task_struct for the process and place is into @secid.
+ * @task_getsecid_subj:
+ *	Retrieve the subjective security identifier of the task_struct in @p
+ *	and return it in @secid.  Special care must be taken to ensure that @p
+ *	is the either the "current" task, or the caller has exclusive access
+ *	to @p.
+ *	In case of failure, @secid will be set to zero.
+ * @task_getsecid_obj:
+ *	Retrieve the objective security identifier of the task_struct in @p
+ *	and return it in @secid.
  *	In case of failure, @secid will be set to zero.
  *
  * @task_setnice:

include/linux/security.h

@@ -415,7 +415,8 @@ int security_task_fix_setgid(struct cred *new, const struct cred *old,
 int security_task_setpgid(struct task_struct *p, pid_t pgid);
 int security_task_getpgid(struct task_struct *p);
 int security_task_getsid(struct task_struct *p);
-void security_task_getsecid(struct task_struct *p, u32 *secid);
+void security_task_getsecid_subj(struct task_struct *p, u32 *secid);
+void security_task_getsecid_obj(struct task_struct *p, u32 *secid);
 int security_task_setnice(struct task_struct *p, int nice);
 int security_task_setioprio(struct task_struct *p, int ioprio);
 int security_task_getioprio(struct task_struct *p);
@@ -1106,7 +1107,12 @@ static inline int security_task_getsid(struct task_struct *p)
 	return 0;
 }
 
-static inline void security_task_getsecid(struct task_struct *p, u32 *secid)
+static inline void security_task_getsecid_subj(struct task_struct *p, u32 *secid)
+{
+	*secid = 0;
+}
+
+static inline void security_task_getsecid_obj(struct task_struct *p, u32 *secid)
 {
 	*secid = 0;
 }

kernel/audit.c

@@ -2132,7 +2132,7 @@ int audit_log_task_context(struct audit_buffer *ab)
 	int error;
 	u32 sid;
 
-	security_task_getsecid(current, &sid);
+	security_task_getsecid_subj(current, &sid);
 	if (!sid)
 		return 0;
 
@@ -2353,7 +2353,7 @@ int audit_signal_info(int sig, struct task_struct *t)
 			audit_sig_uid = auid;
 		else
 			audit_sig_uid = uid;
-		security_task_getsecid(current, &audit_sig_sid);
+		security_task_getsecid_subj(current, &audit_sig_sid);
 	}
 
 	return audit_signal_info_syscall(t);

kernel/auditfilter.c

@@ -1359,7 +1359,8 @@ int audit_filter(int msgtype, unsigned int listtype)
 			case AUDIT_SUBJ_SEN:
 			case AUDIT_SUBJ_CLR:
 				if (f->lsm_rule) {
-					security_task_getsecid(current, &sid);
+					security_task_getsecid_subj(current,
+								    &sid);
 					result = security_audit_rule_match(sid,
 						   f->type, f->op, f->lsm_rule);
 				}

kernel/auditsc.c

@@ -667,7 +667,7 @@ static int audit_filter_rules(struct task_struct *tsk,
 			   logged upon error */
 			if (f->lsm_rule) {
 				if (need_sid) {
-					security_task_getsecid(tsk, &sid);
+					security_task_getsecid_subj(tsk, &sid);
 					need_sid = 0;
 				}
 				result = security_audit_rule_match(sid, f->type,
@@ -2400,7 +2400,7 @@ void __audit_ptrace(struct task_struct *t)
 	context->target_auid = audit_get_loginuid(t);
 	context->target_uid = task_uid(t);
 	context->target_sessionid = audit_get_sessionid(t);
-	security_task_getsecid(t, &context->target_sid);
+	security_task_getsecid_obj(t, &context->target_sid);
 	memcpy(context->target_comm, t->comm, TASK_COMM_LEN);
 }
 
@@ -2427,7 +2427,7 @@ int audit_signal_info_syscall(struct task_struct *t)
 		ctx->target_auid = audit_get_loginuid(t);
 		ctx->target_uid = t_uid;
 		ctx->target_sessionid = audit_get_sessionid(t);
-		security_task_getsecid(t, &ctx->target_sid);
+		security_task_getsecid_obj(t, &ctx->target_sid);
 		memcpy(ctx->target_comm, t->comm, TASK_COMM_LEN);
 		return 0;
 	}
@@ -2448,7 +2448,7 @@ int audit_signal_info_syscall(struct task_struct *t)
 	axp->target_auid[axp->pid_count] = audit_get_loginuid(t);
 	axp->target_uid[axp->pid_count] = t_uid;
 	axp->target_sessionid[axp->pid_count] = audit_get_sessionid(t);
-	security_task_getsecid(t, &axp->target_sid[axp->pid_count]);
+	security_task_getsecid_obj(t, &axp->target_sid[axp->pid_count]);
 	memcpy(axp->target_comm[axp->pid_count], t->comm, TASK_COMM_LEN);
 	axp->pid_count++;
 

kernel/bpf/bpf_lsm.c

@@ -209,7 +209,8 @@ BTF_ID(func, bpf_lsm_socket_socketpair)
 
 BTF_ID(func, bpf_lsm_syslog)
 BTF_ID(func, bpf_lsm_task_alloc)
-BTF_ID(func, bpf_lsm_task_getsecid)
+BTF_ID(func, bpf_lsm_task_getsecid_subj)
+BTF_ID(func, bpf_lsm_task_getsecid_obj)
 BTF_ID(func, bpf_lsm_task_prctl)
 BTF_ID(func, bpf_lsm_task_setscheduler)
 BTF_ID(func, bpf_lsm_task_to_inode)

net/netlabel/netlabel_unlabeled.c

@@ -1539,7 +1539,7 @@ int __init netlbl_unlabel_defconf(void)
 	/* Only the kernel is allowed to call this function and the only time
 	 * it is called is at bootup before the audit subsystem is reporting
 	 * messages so don't worry to much about these values. */
-	security_task_getsecid(current, &audit_info.secid);
+	security_task_getsecid_subj(current, &audit_info.secid);
 	audit_info.loginuid = GLOBAL_ROOT_UID;
 	audit_info.sessionid = 0;