Remove world-writability from per-user directories

'nix-daemon' now creates subdirectories for users when they first
connect.

Fixes #509 (CVE-2019-17365).
Should also fix #3127.

nix.spec.in

@@ -106,7 +106,7 @@ chmod 1775 $RPM_BUILD_ROOT/nix/store
 for d in profiles gcroots;
 do
   mkdir -p $RPM_BUILD_ROOT/nix/var/nix/$d/per-user
-  chmod 1777 $RPM_BUILD_ROOT/nix/var/nix/$d/per-user
+  chmod 755 $RPM_BUILD_ROOT/nix/var/nix/$d/per-user
 done
 
 # fix permission of nix profile

scripts/install-multi-user.sh

@@ -437,16 +437,15 @@ create_build_users() {
 }
 
 create_directories() {
+    # FIXME: remove all of this because it duplicates LocalStore::LocalStore().
+
     _sudo "to make the basic directory structure of Nix (part 1)" \
-          mkdir -pv -m 0755 /nix /nix/var /nix/var/log /nix/var/log/nix /nix/var/log/nix/drvs /nix/var/nix{,/db,/gcroots,/profiles,/temproots,/userpool}
+          mkdir -pv -m 0755 /nix /nix/var /nix/var/log /nix/var/log/nix /nix/var/log/nix/drvs /nix/var/nix{,/db,/gcroots,/profiles,/temproots,/userpool} /nix/var/nix/{gcroots,profiles}/per-user
 
     _sudo "to make the basic directory structure of Nix (part 2)" \
-          mkdir -pv -m 1777 /nix/var/nix/{gcroots,profiles}/per-user
-
-    _sudo "to make the basic directory structure of Nix (part 3)" \
           mkdir -pv -m 1775 /nix/store
 
-    _sudo "to make the basic directory structure of Nix (part 4)" \
+    _sudo "to make the basic directory structure of Nix (part 3)" \
           chgrp "$NIX_BUILD_GROUP_NAME" /nix/store
 
     _sudo "to set up the root user's profile (part 1)" \

scripts/nix-profile-daemon.sh.in

@@ -5,12 +5,6 @@ __ETC_PROFILE_NIX_SOURCED=1
 export NIX_USER_PROFILE_DIR="@localstatedir@/nix/profiles/per-user/$USER"
 export NIX_PROFILES="@localstatedir@/nix/profiles/default $HOME/.nix-profile"
 
-# Set up the per-user profile.
-mkdir -m 0755 -p $NIX_USER_PROFILE_DIR
-if ! test -O "$NIX_USER_PROFILE_DIR"; then
-    echo "WARNING: bad ownership on $NIX_USER_PROFILE_DIR" >&2
-fi
-
 if test -w $HOME; then
   if ! test -L $HOME/.nix-profile; then
       if test "$USER" != root; then
@@ -26,13 +20,6 @@ if test -w $HOME; then
       echo "https://nixos.org/channels/nixpkgs-unstable nixpkgs" > $HOME/.nix-channels
   fi
 
-  # Create the per-user garbage collector roots directory.
-  NIX_USER_GCROOTS_DIR=@localstatedir@/nix/gcroots/per-user/$USER
-  mkdir -m 0755 -p $NIX_USER_GCROOTS_DIR
-  if ! test -O "$NIX_USER_GCROOTS_DIR"; then
-      echo "WARNING: bad ownership on $NIX_USER_GCROOTS_DIR" >&2
-  fi
-
   # Set up a default Nix expression from which to install stuff.
   if [ ! -e $HOME/.nix-defexpr -o -L $HOME/.nix-defexpr ]; then
       rm -f $HOME/.nix-defexpr

scripts/nix-profile.sh.in

@@ -9,12 +9,6 @@ if [ -n "$HOME" ] && [ -n "$USER" ]; then
 
     NIX_USER_PROFILE_DIR=@localstatedir@/nix/profiles/per-user/$USER
 
-    mkdir -m 0755 -p "$NIX_USER_PROFILE_DIR"
-
-    if [ "$(stat --printf '%u' "$NIX_USER_PROFILE_DIR")" != "$(id -u)" ]; then
-        echo "Nix: WARNING: bad ownership on "$NIX_USER_PROFILE_DIR", should be $(id -u)" >&2
-    fi
-
     if [ -w "$HOME" ]; then
         if ! [ -L "$NIX_LINK" ]; then
             echo "Nix: creating $NIX_LINK" >&2
@@ -33,14 +27,6 @@ if [ -n "$HOME" ] && [ -n "$USER" ]; then
             echo "https://nixos.org/channels/nixpkgs-unstable nixpkgs" > "$HOME/.nix-channels"
         fi
 
-        # Create the per-user garbage collector roots directory.
-        __user_gcroots=@localstatedir@/nix/gcroots/per-user/"$USER"
-        mkdir -m 0755 -p "$__user_gcroots"
-        if [ "$(stat --printf '%u' "$__user_gcroots")" != "$(id -u)" ]; then
-            echo "Nix: WARNING: bad ownership on $__user_gcroots, should be $(id -u)" >&2
-        fi
-        unset __user_gcroots
-
         # Set up a default Nix expression from which to install stuff.
         __nix_defexpr="$HOME"/.nix-defexpr
         [ -L "$__nix_defexpr" ] && rm -f "$__nix_defexpr"

src/libstore/local-store.cc

@@ -74,10 +74,11 @@ LocalStore::LocalStore(const Params & params)
        multi-user install. */
     if (getuid() == 0 && settings.buildUsersGroup != "") {
 
-        Path perUserDir = profilesDir + "/per-user";
+        for (auto & perUserDir : {profilesDir + "/per-user", gcRootsDir + "/per-user"}) {
             createDirs(perUserDir);
-        if (chmod(perUserDir.c_str(), 01777) == -1)
+            if (chmod(perUserDir.c_str(), 0755) == -1)
-            throw SysError(format("could not set permissions on '%1%' to 1777") % perUserDir);
+                throw SysError("could not set permissions on '%s' to 755", perUserDir);
+        }
 
         mode_t perm = 01775;
 
@@ -1432,4 +1433,19 @@ void LocalStore::signPathInfo(ValidPathInfo & info)
 }
 
 
+void LocalStore::createUser(const std::string & userName, uid_t userId)
+{
+    for (auto & dir : {
+        fmt("%s/profiles/per-user/%s", stateDir, userName),
+        fmt("%s/gcroots/per-user/%s", stateDir, userName)
+    }) {
+        createDirs(dir);
+        if (chmod(dir.c_str(), 0700) == -1)
+            throw SysError("changing permissions of directory '%s'", dir);
+        if (chown(dir.c_str(), userId, -1) == -1)
+            throw SysError("changing owner of directory '%s'", dir);
+    }
+}
+
+
 }

src/libstore/local-store.hh

@@ -293,6 +293,8 @@ private:
 
     Path getRealStoreDir() override { return realStoreDir; }
 
+    void createUser(const std::string & userName, uid_t userId) override;
+
     friend class DerivationGoal;
     friend class SubstitutionGoal;
 };

src/libstore/store-api.hh

@@ -628,6 +628,9 @@ public:
         return storePath;
     }
 
+    virtual void createUser(const std::string & userName, uid_t userId)
+    { }
+
 protected:
 
     Stats stats;

src/nix-daemon/nix-daemon.cc

@@ -742,7 +742,8 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
 }
 
 
-static void processConnection(bool trusted)
+static void processConnection(bool trusted,
+    const std::string & userName, uid_t userId)
 {
     MonitorFdHup monitor(from.fd);
 
@@ -793,6 +794,8 @@ static void processConnection(bool trusted)
         params["path-info-cache-size"] = "0";
         auto store = openStore(settings.storeUri, params);
 
+        store->createUser(userName, userId);
+
         tunnelLogger->stopWork();
         to.flush();
 
@@ -1053,7 +1056,7 @@ static void daemonLoop(char * * argv)
                 /* Handle the connection. */
                 from.fd = remote.get();
                 to.fd = remote.get();
-                processConnection(trusted);
+                processConnection(trusted, user, peer.uid);
 
                 exit(0);
             }, options);
@@ -1133,7 +1136,7 @@ static int _main(int argc, char * * argv)
                     }
                 }
             } else {
-                processConnection(true);
+                processConnection(true, "root", 0);
             }
         } else {
             daemonLoop(argv);

tests/nix-profile.sh

@@ -10,5 +10,3 @@ USER=$user $SHELL -e -c ". $TEST_ROOT/nix-profile.sh" # test idempotency
 
 [ -L $TEST_HOME/.nix-profile ]
 [ -e $TEST_HOME/.nix-channels ]
-[ -e $TEST_ROOT/profile-var/nix/gcroots/per-user/$user ]
-[ -e $TEST_ROOT/profile-var/nix/profiles/per-user/$user ]

tests/remote-store.sh

@@ -13,3 +13,7 @@ cmp $TEST_ROOT/d1 $TEST_ROOT/d2
 nix-store --gc --max-freed 1K
 
 killDaemon
+
+user=$(whoami)
+[ -e $NIX_STATE_DIR/gcroots/per-user/$user ]
+[ -e $NIX_STATE_DIR/profiles/per-user/$user ]

tests/user-envs.sh

@@ -20,7 +20,7 @@ drvPath10=$(nix-env -f ./user-envs.nix -qa --drv-path --no-name '*' | grep foo-1
 
 # Query descriptions.
 nix-env -f ./user-envs.nix -qa '*' --description | grep -q silly
-rm -f $HOME/.nix-defexpr
+rm -rf $HOME/.nix-defexpr
 ln -s $(pwd)/user-envs.nix $HOME/.nix-defexpr
 nix-env -qa '*' --description | grep -q silly