security: Add LSM hooks to set*gid syscalls

The SafeSetID LSM uses the security_task_fix_setuid hook to filter
set*uid() syscalls according to its configured security policy. In
preparation for adding analagous support in the LSM for set*gid()
syscalls, we add the requisite hook here. Tested by putting print
statements in the security_task_fix_setgid hook and seeing them get hit
during kernel boot.
Signed-off-by: Thomas Cedeno <thomascedeno@google.com>
Signed-off-by: Micah Morton <mortonm@chromium.org>

include/linux/lsm_hook_defs.h

@@ -190,6 +190,8 @@ LSM_HOOK(int, 0, kernel_post_read_file, struct file *file, char *buf,
 	 loff_t size, enum kernel_read_file_id id)
 LSM_HOOK(int, 0, task_fix_setuid, struct cred *new, const struct cred *old,
 	 int flags)
+LSM_HOOK(int, 0, task_fix_setgid, struct cred *new, const struct cred * old,
+	 int flags)
 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)

include/linux/lsm_hooks.h

@@ -651,6 +651,15 @@
  *	@old is the set of credentials that are being replaces
  *	@flags contains one of the LSM_SETID_* values.
  *	Return 0 on success.
+ * @task_fix_setgid:
+ *	Update the module's state after setting one or more of the group
+ *	identity attributes of the current process.  The @flags parameter
+ *	indicates which of the set*gid system calls invoked this hook.
+ *	@new is the set of credentials that will be installed.  Modifications
+ *	should be made to this rather than to @current->cred.
+ *	@old is the set of credentials that are being replaced.
+ *	@flags contains one of the LSM_SETID_* values.
+ *	Return 0 on success.
  * @task_setpgid:
  *	Check permission before setting the process group identifier of the
  *	process @p to @pgid.

include/linux/security.h

@@ -389,6 +389,8 @@ int security_kernel_post_read_file(struct file *file, char *buf, loff_t size,
 				   enum kernel_read_file_id id);
 int security_task_fix_setuid(struct cred *new, const struct cred *old,
 			     int flags);
+int security_task_fix_setgid(struct cred *new, const struct cred *old,
+			     int flags);
 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);
@@ -1027,6 +1029,13 @@ static inline int security_task_fix_setuid(struct cred *new,
 	return cap_task_fix_setuid(new, old, flags);
 }
 
+static inline int security_task_fix_setgid(struct cred *new,
+					   const struct cred *old,
+					   int flags)
+{
+	return 0;
+}
+
 static inline int security_task_setpgid(struct task_struct *p, pid_t pgid)
 {
 	return 0;

kernel/sys.c

@@ -393,6 +393,10 @@ long __sys_setregid(gid_t rgid, gid_t egid)
 		new->sgid = new->egid;
 	new->fsgid = new->egid;
 
+	retval = security_task_fix_setgid(new, old, LSM_SETID_RE);
+	if (retval < 0)
+		goto error;
+
 	return commit_creds(new);
 
 error:
@@ -435,6 +439,10 @@ long __sys_setgid(gid_t gid)
 	else
 		goto error;
 
+	retval = security_task_fix_setgid(new, old, LSM_SETID_ID);
+	if (retval < 0)
+		goto error;
+
 	return commit_creds(new);
 
 error:
@@ -756,6 +764,10 @@ long __sys_setresgid(gid_t rgid, gid_t egid, gid_t sgid)
 		new->sgid = ksgid;
 	new->fsgid = new->egid;
 
+	retval = security_task_fix_setgid(new, old, LSM_SETID_RES);
+	if (retval < 0)
+		goto error;
+
 	return commit_creds(new);
 
 error:
@@ -862,6 +874,7 @@ long __sys_setfsgid(gid_t gid)
 	    ns_capable(old->user_ns, CAP_SETGID)) {
 		if (!gid_eq(kgid, old->fsgid)) {
 			new->fsgid = kgid;
+			if (security_task_fix_setgid(new,old,LSM_SETID_FS) == 0)
 				goto change_okay;
 		}
 	}

security/security.c

@@ -1685,6 +1685,12 @@ int security_task_fix_setuid(struct cred *new, const struct cred *old,
 	return call_int_hook(task_fix_setuid, 0, new, old, flags);
 }
 
+int security_task_fix_setgid(struct cred *new, const struct cred *old,
+				 int flags)
+{
+	return call_int_hook(task_fix_setgid, 0, new, old, flags);
+}
+
 int security_task_setpgid(struct task_struct *p, pid_t pgid)
 {
 	return call_int_hook(task_setpgid, 0, p, pgid);