[RFC PATCH 8/9] selinux: add support for the io_uring access controls

[Paul Moore]

WARNING - This is a work in progress, this patch, including the
description, may be incomplete or even incorrect.  You have been
warned.

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 at paul-moore.com>
---
 security/selinux/hooks.c            |   67 +++++++++++++++++++++++++++++++++++
 security/selinux/include/classmap.h |    2 +
 2 files changed, 69 insertions(+)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index eaea837d89d1..696130972e4d 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -7115,6 +7115,68 @@ static int selinux_perf_event_write(struct perf_event *event)
 }
 #endif
 
+#ifdef CONFIG_IO_URING
+
+#if 1
+/* XXX - dump_creds() is for debugging only, remove before committing */
+static void dump_creds(const struct cred *cred)
+{
+	u32 secid;
+	char *ctx = NULL;
+	unsigned int ctx_len = 0;
+
+	security_cred_getsecid(cred, &secid);
+	if (secid)
+		security_secid_to_secctx(secid, &ctx, &ctx_len);
+
+	printk(KERN_ERR "CRED: ->security is %s\n", (ctx ? : "(error)"));
+
+	if (ctx)
+		security_release_secctx(ctx, ctx_len);
+}
+#endif
+
+/**
+ * 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.
+ */
+int selinux_uring_override_creds(const struct cred *new)
+{
+#if 1
+	/* XXX - debug only code, remove before committing */
+	pr_err("PMD: selinux_uring_override_creds()\n");
+	pr_err("PMD: >>> CURRENT\n");
+	dump_creds(current_cred());
+	pr_err("PMD: >>> NEW\n");
+	dump_creds(new);
+#endif
+	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.
+ */
+int selinux_uring_sqpoll(void)
+{
+	int sid = current_sid();
+#if 1
+	/* XXX - debug only code, remove before committing */
+	pr_err("PMD: selinux_uring_sqpoll()\n");
+	pr_err("PMD: >>> CURRENT\n");
+	dump_creds(current_cred());
+#endif
+	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,
@@ -7353,6 +7415,11 @@ 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),
 
 	/*
diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h
index 62d19bccf3de..3314ad72279d 100644
--- a/security/selinux/include/classmap.h
+++ b/security/selinux/include/classmap.h
@@ -252,6 +252,8 @@ struct security_class_mapping secclass_map[] = {
 	  { "integrity", "confidentiality", NULL } },
 	{ "anon_inode",
 	  { COMMON_FILE_PERMS, NULL } },
+	{ "io_uring",
+	  { "override_creds", "sqpoll", NULL } },
 	{ NULL }
   };