[PATCH] support_become_root: Enable file creation in user namespaces

[fweimer at redhat dot com (Florian Weimer)]

Without UID/GID maps, file creation will file with EOVERFLOW.

This patch is based on DJ Delorie's work on container testing.

2017-11-17  Florian Weimer  <fweimer@redhat.com>

	support_become_root: Enable file creation in namespaces.
	* support/support_become_root.c (setup_mapping): New function.
	(support_become_root): Call it.

diff --git a/support/support_become_root.c b/support/support_become_root.c
index 3fa0bd4ac0..5086570251 100644
--- a/support/support_become_root.c
+++ b/support/support_become_root.c
@@ -18,18 +18,69 @@
 
 #include <support/namespace.h>
 
+#include <fcntl.h>
 #include <sched.h>
 #include <stdio.h>
+#include <string.h>
+#include <support/check.h>
+#include <support/xunistd.h>
 #include <unistd.h>
 
+#ifdef CLONE_NEWUSER
+/* The necessary steps to allow file creation in user namespaces.  */
+static void
+setup_uid_gid_mapping (uid_t original_uid, gid_t original_gid)
+{
+  int fd = open64 ("/proc/self/uid_map", O_WRONLY);
+  if (fd < 0)
+    {
+      printf ("warning: could not open /proc/self/uid_map: %m\n"
+              "warning: file creation may not be possible\n");
+      return;
+    }
+
+  /* We map our original UID to the same UID in the container so we
+     own our own files normally.  Without that, file creation could
+     fail with EOVERFLOW (sic!).  */
+  char buf[100];
+  int ret = snprintf (buf, sizeof (buf), "%llu %llu 1\n",
+                      (unsigned long long) original_uid,
+                      (unsigned long long) original_uid);
+  TEST_VERIFY_EXIT (ret < sizeof (buf));
+  xwrite (fd, buf, ret);
+  xclose (fd);
+
+  /* Disable setgroups before mapping groups, otherwise that would
+     fail with EPERM.  */
+  fd = xopen ("/proc/self/setgroups", O_WRONLY, 0);
+  xwrite (fd, "deny\n", strlen ("deny\n"));
+  xclose (fd);
+
+  /* Now map our own GID, like we did for the user ID.  */
+  fd = xopen ("/proc/self/gid_map", O_WRONLY, 0);
+  ret = snprintf (buf, sizeof (buf), "%llu %llu 1\n",
+                  (unsigned long long) original_gid,
+                  (unsigned long long) original_gid);
+  TEST_VERIFY_EXIT (ret < sizeof (buf));
+  xwrite (fd, buf, ret);
+  xclose (fd);
+}
+#endif /* CLONE_NEWUSER */
+
 bool
 support_become_root (void)
 {
 #ifdef CLONE_NEWUSER
+  uid_t original_uid = getuid ();
+  gid_t original_gid = getgid ();
+
   if (unshare (CLONE_NEWUSER | CLONE_NEWNS) == 0)
-    /* Even if we do not have UID zero, we have extended privileges at
-       this point.  */
-    return true;
+    {
+      setup_uid_gid_mapping (original_uid, original_gid);
+      /* Even if we do not have UID zero, we have extended privileges at
+         this point.  */
+      return true;
+    }
 #endif
   if (setuid (0) != 0)
     {