daemon: Allow running as non-root with unprivileged user namespaces.

Many thanks to Reepca Russelstein for their review and guidance on these
changes.

* nix/libstore/build.cc (guestUID, guestGID): New variables.
(DerivationGoal)[readiness]: New field.
(initializeUserNamespace): New function.
(DerivationGoal::runChild): When ‘readiness.readSide’ is positive, read
from it.
(DerivationGoal::startBuilder): Call ‘chown’
only when ‘buildUser.enabled()’ is true.  Pass CLONE_NEWUSER to ‘clone’
when ‘buildUser.enabled()’ is false or not running as root.  Retry
‘clone’ without CLONE_NEWUSER upon EPERM.
(DerivationGoal::registerOutputs): Make ‘actualPath’ writable before
‘rename’.
(DerivationGoal::deleteTmpDir): Catch ‘SysError’ around ‘_chown’ call.
* nix/libstore/local-store.cc (LocalStore::createUser): Do nothing if
‘dirs’ already exists.  Warn instead of failing when failing to chown
‘dir’.
* guix/substitutes.scm (%narinfo-cache-directory): Check for
‘_NIX_OPTIONS’ rather than getuid() == 0 to determine the cache
location.
* doc/guix.texi (Build Environment Setup): Reorganize a bit.  Add
section headings “Daemon Running as Root” and “The Isolated Build
Environment”.  Add “Daemon Running Without Privileges” subsection.
Remove paragraph about ‘--disable-chroot’.
(Invoking guix-daemon): Warn against ‘--disable-chroot’ and explain why.
* tests/derivations.scm ("builder is outside the store"): New test.

Reviewed-by: Reepca Russelstein <reepca@russelstein.xyz>

tests/derivations.scm

@@ -858,6 +858,20 @@
          (call-with-input-file (derivation->output-path drv)
            get-string-all))))
 
+(test-assert "builder is outside the store"
+  ;; Ensure that attempts to build derivations whose builder is outside the
+  ;; store are rejected.  This is a protection against attacks similar to
+  ;; CVE-2019-5736, which abuse the fact that /proc/self/exe can be opened
+  ;; even when it presents itself as a symlink to a file not in the chroot.
+  (let* ((builder (add-file-tree-to-store %store
+                                          `("builder" symlink "/proc/self/exe")))
+         (drv (derivation %store "attempt-to-run-guix-daemon" builder '()
+                          #:env-vars
+                          '(("LD_PRELOAD" . "attacker-controlled.so")))))
+    (guard (c ((store-protocol-error? c) c))
+      (build-derivations %store (list drv))
+      #f)))
+
 
 (define %coreutils
   (false-if-exception