Subject: [PATCH v2 7/9] daemon: Drop Linux ambient capabilities before executing builder.

From: Ludovic Courtès <[email-masked]>
Date: Thu, 13 Feb 2025 13:13:10 +0100

* config-daemon.ac: Check for <sys/prctl.h>.
* nix/libstore/build.cc (DerivationGoal::runChild): When ‘useChroot’ is
true, call ‘prctl’ to drop all ambient capabilities.

Change-Id: If34637fc508e5fb6d278167f5df7802fc595284f
---
 config-daemon.ac      | 2 +-
 nix/libstore/build.cc | 9 +++++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/config-daemon.ac b/config-daemon.ac
index 4e949bc88a3..35d9c8cd56b 100644
--- a/config-daemon.ac
+++ b/config-daemon.ac
@@ -79,7 +79,7 @@ if test "x$guix_build_daemon" = "xyes"; then
   dnl Chroot support.
   AC_CHECK_FUNCS([chroot unshare])
   AC_CHECK_HEADERS([sched.h sys/param.h sys/mount.h sys/syscall.h \
-    linux/close_range.h])
+    linux/close_range.h sys/prctl.h])
 
   if test "x$ac_cv_func_chroot" != "xyes"; then
     AC_MSG_ERROR(['chroot' function missing, bailing out])
diff --git a/nix/libstore/build.cc b/nix/libstore/build.cc
index 18dd27460b7..4280b8abff8 100644
--- a/nix/libstore/build.cc
+++ b/nix/libstore/build.cc
@@ -50,6 +50,9 @@
 #if HAVE_SCHED_H
 #include <sched.h>
 #endif
+#if HAVE_SYS_PRCTL_H
+#include <sys/prctl.h>
+#endif

 #define CHROOT_ENABLED HAVE_CHROOT && HAVE_SYS_MOUNT_H && defined(MS_BIND) && defined(MS_PRIVATE)
@@ -2059,6 +2062,12 @@ void DerivationGoal::runChild()
 
 #if CHROOT_ENABLED
         if (useChroot) {
+# if HAVE_SYS_PRCTL_H
+	    /* Drop ambient capabilities such as CAP_CHOWN that might have
+	       been granted when starting guix-daemon.  */
+	    prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_CLEAR_ALL, 0, 0, 0);
+# endif
+
 	    if (!fixedOutput) {
 		/* Initialise the loopback interface. */
 		AutoCloseFD fd(socket(PF_INET, SOCK_DGRAM, IPPROTO_IP));
-- 
2.48.1