* 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.

2 files changed, 88 insertions, 0 deletions

diff --git a/src/Makefile.am b/src/Makefile.am
index 29bd535f6..6c3e5ee20 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 3f9b2a10d..068c12659 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);