* Setuid support for sharing a Nix installation between multiple

  users.

  If the configure flag `--enable-setuid' is used, the Nix programs
  nix-env, nix-store, etc. are installed with the setuid bit turned on
  so that they are executed as the user and group specified by
  `--with-nix-user=USER' and `--with-nix-group=GROUP', respectively
  (with defaults `nix' and `nix').

  The setuid programs drop all special privileges if they are executed
  by a user who is not a member of the Nix group.

  The setuid feature is a quick hack to enable sharing of a Nix
  installation between users who trust each other.  It is not
  generally secure, since any user in the Nix group can modify (by
  building an appropriate derivation) any object in the store, and for
  instance inject trojans into binaries used by other users.

  The setuid programs are owned by root, not the Nix user.  This is
  because on Unix normal users cannot change the real uid, only the
  effective uid.  Many programs don't work properly when the real uid
  differs from the effective uid.  For instance, Perl will turn on
  taint mode.  However, the setuid programs drop all root privileges
  immediately, changing all uids and gids to the Nix user and group.

5 files changed, 121 insertions, 8 deletions

diff --git a/Makefile.am b/Makefile.am
index 2213a81ddd5d..9f9f9e0a9386 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -14,16 +14,19 @@ relname:
 install-data-local: init-state
 
 if INIT_STATE
+if SETUID_HACK
+INIT_FLAGS = -g @NIX_GROUP@ -o @NIX_USER@
+endif
 init-state:
-	$(INSTALL) -d $(DESTDIR)$(localstatedir)/nix
-	$(INSTALL) -d $(DESTDIR)$(localstatedir)/nix/db
-	$(INSTALL) -d $(DESTDIR)$(localstatedir)/log/nix
-	$(INSTALL) -d $(DESTDIR)$(localstatedir)/nix/profiles
-	$(INSTALL) -d $(DESTDIR)$(localstatedir)/nix/gcroots
-	$(INSTALL) -d $(DESTDIR)$(localstatedir)/nix/gcroots/tmp
+	$(INSTALL) $(INIT_FLAGS) -d $(DESTDIR)$(localstatedir)/nix
+	$(INSTALL) $(INIT_FLAGS) -d $(DESTDIR)$(localstatedir)/nix/db
+	$(INSTALL) $(INIT_FLAGS) -d $(DESTDIR)$(localstatedir)/log/nix
+	$(INSTALL) $(INIT_FLAGS) -d $(DESTDIR)$(localstatedir)/nix/profiles
+	$(INSTALL) $(INIT_FLAGS) -d $(DESTDIR)$(localstatedir)/nix/gcroots
+	$(INSTALL) $(INIT_FLAGS) -d $(DESTDIR)$(localstatedir)/nix/gcroots/tmp
 	rm -f $(DESTDIR)$(localstatedir)/nix/gcroots/profiles
 	ln -s $(localstatedir)/nix/profiles $(DESTDIR)$(localstatedir)/nix/gcroots/profiles
-	$(INSTALL) -d $(DESTDIR)$(prefix)/store
+	$(INSTALL) $(INIT_FLAGS) -d $(DESTDIR)$(prefix)/store
 #	$(bindir)/nix-store --init
 else
 init-state:
diff --git a/configure.ac b/configure.ac
index 0d7045de18f7..5dab9847ad8f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -142,6 +142,28 @@ AC_ARG_ENABLE(init-state, AC_HELP_STRING([--disable-init-state],
   init_state=$enableval, init_state=yes)
 AM_CONDITIONAL(INIT_STATE, test "$init_state" = "yes")
 
+
+AC_ARG_ENABLE(setuid, AC_HELP_STRING([--enable-setuid],
+  [install Nix setuid]),
+  setuid_hack=$enableval, setuid_hack=no)
+AM_CONDITIONAL(SETUID_HACK, test "$setuid_hack" = "yes")
+if test "$setuid_hack" = "yes"; then
+    AC_DEFINE(SETUID_HACK, 1, [whether to install Nix setuid])
+fi
+
+AC_ARG_WITH(nix-user, AC_HELP_STRING([--with-nix-user=USER],
+  [user for Nix setuid binaries]),
+  NIX_USER=$withval, NIX_USER=nix)
+AC_SUBST(NIX_USER)
+AC_DEFINE_UNQUOTED(NIX_USER, ["$NIX_USER"], [Nix user])
+                                                      
+AC_ARG_WITH(nix-group, AC_HELP_STRING([--with-nix-group=USER],
+  [group for Nix setuid binaries]),
+  NIX_GROUP=$withval, NIX_GROUP=nix)
+AC_SUBST(NIX_GROUP)
+AC_DEFINE_UNQUOTED(NIX_GROUP, ["$NIX_GROUP"], [Nix group])
+
+                                                                                                            
 AM_CONFIG_HEADER([config.h])
 AC_CONFIG_FILES([Makefile
    externals/Makefile
diff --git a/scripts/nix-push.in b/scripts/nix-push.in
index 84330016f310..fc44d02c6cba 100644
--- a/scripts/nix-push.in
+++ b/scripts/nix-push.in
@@ -92,7 +92,7 @@ while (scalar @tmp > 0) {
     my @tmp2 = @tmp[0..$n - 1];
     @tmp = @tmp[$n..scalar @tmp - 1];
 
-    system "@bindir@/nix-store --realise -B @tmp2 > /dev/null";
+    system "@bindir@/nix-store --realise @tmp2 > /dev/null";
     if ($?) { die "`nix-store --realise' failed"; }
 
     open NARPATHS, "@bindir@/nix-store --query --list @tmp2 |" or die "cannot run nix";
diff --git a/src/Makefile.am b/src/Makefile.am
index 29bd535f6ee0..6c3e5ee209ae 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -1,2 +1,8 @@
 SUBDIRS = bin2c boost libutil libstore libmain nix-store nix-hash \
  libexpr nix-instantiate nix-env log2xml
+
+SETUID_PROGS = nix-store nix-instantiate nix-env
+install-exec-hook:
+if SETUID_HACK
+	cd $(DESTDIR)$(bindir) && chown root $(SETUID_PROGS) && chmod u+s $(SETUID_PROGS)
+endif
diff --git a/src/libmain/shared.cc b/src/libmain/shared.cc
index 3f9b2a10d4ea..068c126596e7 100644
--- a/src/libmain/shared.cc
+++ b/src/libmain/shared.cc
@@ -5,6 +5,9 @@
 #include <sys/stat.h>
 #include <unistd.h>
 
+#include <pwd.h>
+#include <grp.h>
+
 extern "C" {
 #include <aterm2.h>
 }
@@ -160,10 +163,89 @@ static void initAndRun(int argc, char * * argv)
 }
 
 
+void switchToNixUser()
+{
+#if SETUID_HACK
+
+    /* Here we set the uid and gid to the Nix user and group,
+       respectively, IF the current (real) user is a member of the Nix
+       group.  Otherwise we just drop all privileges. */
+    
+    /* Lookup the Nix gid. */
+    struct group * gr = getgrnam(NIX_GROUP);
+    if (!gr) {
+        cerr << format("missing group `%1%'\n") % NIX_GROUP;
+        exit(1);
+    }
+
+    /* Get the supplementary group IDs for the current user. */
+    int maxGids = 512, nrGids;
+    gid_t gids[maxGids];
+    if ((nrGids = getgroups(maxGids, gids)) == -1) {
+        cerr << format("unable to query gids\n");
+        exit(1);
+    }
+
+    /* Check that the current user is a member of the Nix group. */
+    bool found = false;
+    for (int i = 0; i < nrGids; ++i)
+        if (gids[i] == gr->gr_gid) {
+            found = true;
+            break;
+        }
+
+    if (!found) {
+        /* Not in the Nix group - drop all root/Nix privileges. */
+        setgid(getgid());
+        setuid(getuid());
+        return;
+    }
+
+    /* Set the real, effective and saved gids to gr->gr_gid.  Also
+       make very sure that this succeeded.  We switch the gid first
+       because we cannot do it after we have dropped root uid. */
+    if (setgid(gr->gr_gid) != 0 ||
+        getgid() != gr->gr_gid ||
+        getegid() != gr->gr_gid)
+    {
+        cerr << format("unable to set gid to `%1%'\n") % NIX_GROUP;
+        exit(1);
+    }
+
+    /* Lookup the Nix uid. */
+    struct passwd * pw = getpwnam(NIX_USER);
+    if (!pw) {
+        cerr << format("missing user `%1%'\n") % NIX_USER;
+        exit(1);
+    }
+
+    /* This will drop all root privileges, setting the real, effective
+       and saved uids to pw->pw_uid.  Also make very sure that this
+       succeeded.*/
+    if (setuid(pw->pw_uid) != 0 ||
+        getuid() != pw->pw_uid ||
+        geteuid() != pw->pw_uid)
+    {
+        cerr << format("unable to set uid to `%1%'\n") % NIX_USER;
+        exit(1);
+    }
+
+#endif
+}
+
+
 static char buf[1024];
 
 int main(int argc, char * * argv)
 {
+    /* If we are setuid root, we have to get rid of the excess
+       privileges ASAP. */
+    printMsg(lvlError, format("%1% %2% %3% %4%\n") % getuid() % geteuid()
+        % getgid() % getegid());
+    switchToNixUser();
+    printMsg(lvlError, format("%1% %2% %3% %4%\n") % getuid() % geteuid()
+        % getgid() % getegid());
+    
     /* ATerm setup. */
     ATerm bottomOfStack;
     ATinit(argc, argv, &bottomOfStack);