daemon: Use slirp4netns to provide networking to fixed-output derivations.

Previously, the builder of a fixed-output derivation could communicate with an
external process via an abstract Unix-domain socket.  In particular, it could
send an open file descriptor to the store, granting write access to some of
its output files in the store provided the derivation build fails—the fix for
CVE-2024-27297 did not address this specific case.  It could also send an open
file descriptor to a setuid program, which could then be executed using
execveat to gain the privileges of the build user.

With this change, fixed-output derivations other than “builtin:download”
and “builtin:git-download” always run in a separate network namespace
and have network access provided by a TAP device backed by slirp4netns,
thereby closing the abstract Unix-domain socket channel.

* nix/libstore/globals.hh (Settings)[useHostLoopback, slirp4netns]: new
fields.
* config-daemon.ac (SLIRP4NETNS): new C preprocessor definition.
* nix/libstore/globals.cc (Settings::Settings): initialize them to defaults.
* nix/nix-daemon/guix-daemon.cc (options): add --isolate-host-loopback option.
* doc/guix.texi: document it.
* nix/libstore/build.cc (DerivationGoal)[slirp]: New field.
(setupTap, setupTapAction, waitForSlirpReadyAction, enableRouteLocalnetAction,
 prepareSlirpChrootAction, spawnSlirp4netns, haveGlobalIPv6Address,
 remapIdsTo0Action): New functions.
(initializeUserNamespace): allow the guest UID and GID to be specified.
(DerivationGoal::killChild): When ‘slirp’ is not -1, call ‘kill’.
(DerivationGoal::startBuilder): Unconditionally add CLONE_NEWNET to FLAGS.
When ‘fixedOutput’ is true, spawn ‘slirp4netns’.
When ‘fixedOutput’ and ‘useChroot’ are true, add setupTapAction,
waitForSlirpReadyAction, and enableRouteLocalnetAction to builder setup
phases.
Create a /etc/resolv.conf for fixed-output derivations that directs them to
slirp4netns's dns address.
When settings.useHostLoopback is true, supply fixed-output derivations with a
/etc/hosts that resolves "localhost" to slirp4netns's address for accessing
the host loopback.
* nix/libutil/util.cc (keepOnExec, decodeOctalEscaped, sendFD, receiveFD,
  findProgram): New functions.
* nix/libutil/util.hh (keepOnExec, decodeOctalEscaped, sendFD, receiveFD,
  findProgram): New declarations.
* gnu/packages/package-management.scm (guix): add slirp4netns input for linux
  targets.
* tests/derivations.scm (builder-network-isolated?): new variable.
  ("fixed-output derivation, network access, localhost", "fixed-output
  derivation, network access, external host"):
  skip test case if fixed output derivations are isolated from the network.

Change-Id: Ia3fea2ab7add56df66800071cf15cdafe7bfab96
Signed-off-by: John Kehayias <john.kehayias@protonmail.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/nix/nix-daemon/guix-daemon.cc b/nix/nix-daemon/guix-daemon.cc
index d7ab9c5e649..30727d55593 100644
--- a/nix/nix-daemon/guix-daemon.cc
+++ b/nix/nix-daemon/guix-daemon.cc
@@ -90,6 +90,7 @@ builds derivations on behalf of its clients.");
 #define GUIX_OPT_MAX_SILENT_TIME 19
 #define GUIX_OPT_LOG_COMPRESSION 20
 #define GUIX_OPT_DISCOVER 21
+#define GUIX_OPT_ISOLATE_HOST_LOOPBACK 22
 
 static const struct argp_option options[] =
   {
@@ -160,6 +161,8 @@ to live outputs") },
       n_("listen for connections on SOCKET") },
     { "debug", GUIX_OPT_DEBUG, 0, 0,
       n_("produce debugging output") },
+    { "isolate-host-loopback", GUIX_OPT_ISOLATE_HOST_LOOPBACK, 0, 0,
+      n_("do not allow fixed-output chroot builds to access the host loopback") },
     { 0, 0, 0, 0, 0 }
   };
 
@@ -294,6 +297,9 @@ parse_opt (int key, char *arg, struct argp_state *state)
     case GUIX_OPT_SYSTEM:
       settings.thisSystem = arg;
       break;
+    case GUIX_OPT_ISOLATE_HOST_LOOPBACK:
+      settings.useHostLoopback = false;
+      break;
     default:
       return (error_t) ARGP_ERR_UNKNOWN;
     }