security: Add LSM hook to setgroups() syscall

Give the LSM framework the ability to filter setgroups() syscalls. There
are already analagous hooks for the set*uid() and set*gid() syscalls.
The SafeSetID LSM will use this new hook to ensure setgroups() calls are
allowed by the installed security policy. Tested by putting print
statement in security_task_fix_setgroups() hook and confirming that it
gets hit when userspace does a setgroups() syscall.

Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Reviewed-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Micah Morton <mortonm@chromium.org>

include/linux/lsm_hook_defs.h

@@ -201,6 +201,7 @@ 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_fix_setgroups, 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)

include/linux/lsm_hooks.h

@@ -702,6 +702,13 @@
  *	@old is the set of credentials that are being replaced.
  *	@flags contains one of the LSM_SETID_* values.
  *	Return 0 on success.
+ * @task_fix_setgroups:
+ *	Update the module's state after setting the supplementary group
+ *	identity attributes of the current process.
+ *	@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.
+ *	Return 0 on success.
  * @task_setpgid:
  *	Check permission before setting the process group identifier of the
  *	process @p to @pgid.

include/linux/security.h

@@ -415,6 +415,7 @@ 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_fix_setgroups(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);
@@ -1098,6 +1099,12 @@ static inline int security_task_fix_setgid(struct cred *new,
 	return 0;
 }
 
+static inline int security_task_fix_setgroups(struct cred *new,
+					   const struct cred *old)
+{
+	return 0;
+}
+
 static inline int security_task_setpgid(struct task_struct *p, pid_t pgid)
 {
 	return 0;

kernel/groups.c

@@ -134,13 +134,26 @@ EXPORT_SYMBOL(set_groups);
 int set_current_groups(struct group_info *group_info)
 {
 	struct cred *new;
+	const struct cred *old;
+	int retval;
 
 	new = prepare_creds();
 	if (!new)
 		return -ENOMEM;
 
+	old = current_cred();
+
 	set_groups(new, group_info);
+
+	retval = security_task_fix_setgroups(new, old);
+	if (retval < 0)
+		goto error;
+
 	return commit_creds(new);
+
+error:
+	abort_creds(new);
+	return retval;
 }
 
 EXPORT_SYMBOL(set_current_groups);

security/security.c

@@ -1803,6 +1803,11 @@ int security_task_fix_setgid(struct cred *new, const struct cred *old,
 	return call_int_hook(task_fix_setgid, 0, new, old, flags);
 }
 
+int security_task_fix_setgroups(struct cred *new, const struct cred *old)
+{
+	return call_int_hook(task_fix_setgroups, 0, new, old);
+}
+
 int security_task_setpgid(struct task_struct *p, pid_t pgid)
 {
 	return call_int_hook(task_setpgid, 0, p, pgid);