isolation: Clarify various self-isolation steps

We have a number of steps of self-isolation scattered across our code.
Improve function names and add comments to make it clearer what the self
isolation model is, what the steps do, and why they happen at the points
they happen.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

isolation.c

@@ -12,6 +12,48 @@
  * Author: Stefano Brivio <sbrivio@redhat.com>
  * Author: David Gibson <david@gibson.dropbear.id.au>
  */
+/**
+ * DOC: Theory of Operation
+ *
+ * For security the passt/pasta process performs a number of
+ * self-isolations steps, dropping capabilities, setting namespaces
+ * and otherwise minimising the impact we can have on the system at
+ * large if we were compromised.
+ *
+ * Obviously we can't isolate ourselves from resources before we've
+ * done anything we need to do with those resources, so we have
+ * multiple stages of self-isolation.  In order these are:
+ *
+ * 1. isolate_initial()
+ * ====================
+ *
+ * Executed immediately after startup, drops capabilities we don't
+ * need at any point during execution (or which we gain back when we
+ * need by joining other namespaces).
+ *
+ * 2. isolate_user()
+ * =================
+ *
+ * Executed once we know what user and user namespace we want to
+ * operate in.  Sets our final UID & GID, and enters the correct user
+ * namespace.
+ *
+ * 3. isolate_prefork()
+ * ====================
+ *
+ * Executed after all setup, but before daemonising (fork()ing into
+ * the background).  Uses mount namespace and pivot_root() to remove
+ * our access to the filesystem.
+ *
+ * 4. isolate_postfork()
+ * =====================
+ *
+ * Executed immediately after daemonizing, but before entering the
+ * actual packet forwarding phase of operation.  Or, if not
+ * daemonizing, immediately after isolate_prefork().  Uses seccomp()
+ * to restrict ourselves to the handful of syscalls we need during
+ * runtime operation.
+ */
 
 #include <errno.h>
 #include <fcntl.h>
@@ -48,7 +90,7 @@
 /**
  * drop_caps() - Drop capabilities we might have except for CAP_NET_BIND_SERVICE
  */
-void drop_caps(void)
+static void drop_caps(void)
 {
 	int i;
 
@@ -60,12 +102,31 @@ void drop_caps(void)
 	}
 }
 
+/**
+ * isolate_initial() - Early, config independent self isolation
+ *
+ * Should:
+ *  - drop unneeded capabilities
+ * Musn't:
+ *  - remove filesytem access (we need to access files during setup)
+ */
+void isolate_initial(void)
+{
+	drop_caps();
+}
+
 /**
  * isolate_user() - Switch to final UID/GID and move into userns
  * @uid:	User ID to run as (in original userns)
  * @gid:	Group ID to run as (in original userns)
  * @use_userns:	Whether to join or create a userns
  * @userns:	userns path to enter, may be empty
+ *
+ * Should:
+ *  - set our final UID and GID
+ *  - enter our final user namespace
+ * Mustn't:
+ *  - remove filesystem access (we need that for further setup)
  */
 void isolate_user(uid_t uid, gid_t gid, bool use_userns, const char *userns)
 {
@@ -135,11 +196,19 @@ void isolate_user(uid_t uid, gid_t gid, bool use_userns, const char *userns)
 }
 
 /**
- * sandbox() - Unshare IPC, mount, PID, UTS, and user namespaces, "unmount" root
+ * isolate_prefork() - Self isolation before daemonizing
+ * @c:		Execution context
  *
  * Return: negative error code on failure, zero on success
+ *
+ * Should:
+ *  - Move us to our own IPC and UTS namespaces
+ *  - Move us to a mount namespace with only an empty directory
+ *  - Drop unneeded capabilities (in the new user namespace)
+ * Mustn't:
+ *  - Remove syscalls we need to daemonise
  */
-int sandbox(struct ctx *c)
+int isolate_prefork(struct ctx *c)
 {
 	int flags = CLONE_NEWIPC | CLONE_NEWNS | CLONE_NEWUTS;
 
@@ -188,13 +257,19 @@ int sandbox(struct ctx *c)
 }
 
 /**
- * seccomp() - Set up seccomp filters depending on mode, won't return on failure
+ * isolate_postfork() - Self isolation after daemonizing
  * @c:		Execution context
+ *
+ * Should:
+ *  - disable core dumps
+ *  - limit to a minimal set of syscalls
  */
-void seccomp(const struct ctx *c)
+void isolate_postfork(const struct ctx *c)
 {
 	struct sock_fprog prog;
 
+	prctl(PR_SET_DUMPABLE, 0);
+
 	if (c->mode == MODE_PASST) {
 		prog.len = (unsigned short)ARRAY_SIZE(filter_passt);
 		prog.filter = filter_passt;

isolation.h

@@ -7,9 +7,9 @@
 #ifndef ISOLATION_H
 #define ISOLATION_H
 
-void drop_caps(void);
+void isolate_initial(void);
 void isolate_user(uid_t uid, gid_t gid, bool use_userns, const char *userns);
-int sandbox(struct ctx *c);
+int isolate_prefork(struct ctx *c);
-void seccomp(const struct ctx *c);
+void isolate_postfork(const struct ctx *c);
 
 #endif /* ISOLATION_H */

passt.c

@@ -185,7 +185,7 @@ int main(int argc, char **argv)
 
 	arch_avx2_exec(argv);
 
-	drop_caps();
+	isolate_initial();
 
 	c.pasta_netns_fd = c.fd_tap = c.fd_tap_listen = -1;
 
@@ -289,7 +289,7 @@ int main(int argc, char **argv)
 		}
 	}
 
-	if (sandbox(&c)) {
+	if (isolate_prefork(&c)) {
 		err("Failed to sandbox process, exiting\n");
 		exit(EXIT_FAILURE);
 	}
@@ -299,9 +299,7 @@ int main(int argc, char **argv)
 	else
 		write_pidfile(pidfile_fd, getpid());
 
-	prctl(PR_SET_DUMPABLE, 0);
+	isolate_postfork(&c);
-
-	seccomp(&c);
 
 	timer_init(&c, &now);