selinux: add support for the io_uring access controls

This patch implements two new io_uring access controls, specifically
support for controlling the io_uring "personalities" and
IORING_SETUP_SQPOLL.  Controlling the sharing of io_urings themselves
is handled via the normal file/inode labeling and sharing mechanisms.

The io_uring { override_creds } permission restricts which domains
the subject domain can use to override it's own credentials.
Granting a domain the io_uring { override_creds } permission allows
it to impersonate another domain in io_uring operations.

The io_uring { sqpoll } permission restricts which domains can create
asynchronous io_uring polling threads.  This is important from a
security perspective as operations queued by this asynchronous thread
inherit the credentials of the thread creator by default; if an
io_uring is shared across process/domain boundaries this could result
in one domain impersonating another.  Controlling the creation of
sqpoll threads, and the sharing of io_urings across processes, allow
policy authors to restrict the ability of one domain to impersonate
another via io_uring.

As a quick summary, this patch adds a new object class with two
permissions:

 io_uring { override_creds sqpoll }

These permissions can be seen in the two simple policy statements
below:

  allow domA_t domB_t : io_uring { override_creds };
  allow domA_t self : io_uring { sqpoll };

Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/hooks.c

@@ -7111,6 +7111,35 @@ static int selinux_perf_event_write(struct perf_event *event)
 }
 #endif
 
+#ifdef CONFIG_IO_URING
+/**
+ * selinux_uring_override_creds - check the requested cred override
+ * @new: the target creds
+ *
+ * Check to see if the current task is allowed to override it's credentials
+ * to service an io_uring operation.
+ */
+static int selinux_uring_override_creds(const struct cred *new)
+{
+	return avc_has_perm(&selinux_state, current_sid(), cred_sid(new),
+			    SECCLASS_IO_URING, IO_URING__OVERRIDE_CREDS, NULL);
+}
+
+/**
+ * selinux_uring_sqpoll - check if a io_uring polling thread can be created
+ *
+ * Check to see if the current task is allowed to create a new io_uring
+ * kernel polling thread.
+ */
+static int selinux_uring_sqpoll(void)
+{
+	int sid = current_sid();
+
+	return avc_has_perm(&selinux_state, sid, sid,
+			    SECCLASS_IO_URING, IO_URING__SQPOLL, NULL);
+}
+#endif /* CONFIG_IO_URING */
+
 /*
  * IMPORTANT NOTE: When adding new hooks, please be careful to keep this order:
  * 1. any hooks that don't belong to (2.) or (3.) below,
@@ -7349,6 +7378,11 @@ static struct security_hook_list selinux_hooks[] __lsm_ro_after_init = {
 	LSM_HOOK_INIT(perf_event_write, selinux_perf_event_write),
 #endif
 
+#ifdef CONFIG_IO_URING
+	LSM_HOOK_INIT(uring_override_creds, selinux_uring_override_creds),
+	LSM_HOOK_INIT(uring_sqpoll, selinux_uring_sqpoll),
+#endif
+
 	LSM_HOOK_INIT(locked_down, selinux_lockdown),
 
 	/*

security/selinux/include/classmap.h

@@ -254,6 +254,8 @@ struct security_class_mapping secclass_map[] = {
 	  { "integrity", "confidentiality", NULL } },
 	{ "anon_inode",
 	  { COMMON_FILE_PERMS, NULL } },
+	{ "io_uring",
+	  { "override_creds", "sqpoll", NULL } },
 	{ NULL }
   };