Subject: [PATCH v4] daemon: Set ownership of kept build directories to the calling user.

From: Hartmut Goebel <[email-masked]>
Date: Thu, 11 Jul 2019 22:26:09 +0200

Fixes <http://bugs.gnu.org/15890>.

* nix/libstore/globals.hh (Settings) Add clientUid and clientGid.
* nix/nix-daemon/nix-daemon.cc (daemonLoop] Store UID and GID of the
  caller in settings.
* nix/libstore/build.cc (_chown): New function.
  (DerivationGoal::deleteTmpDir): Use it, change ownership of build
  directory if it is kept and the new owner is not root.
---
 nix/libstore/build.cc        | 21 +++++++++++++++++++++
 nix/libstore/globals.hh      |  6 ++++++
 nix/nix-daemon/nix-daemon.cc | 12 ++++++++++++
 3 files changed, 39 insertions(+)

diff --git a/nix/libstore/build.cc b/nix/libstore/build.cc
index 889ee3d..e823001 100644
--- a/nix/libstore/build.cc
+++ b/nix/libstore/build.cc
@@ -2631,6 +2631,21 @@ void DerivationGoal::closeLogFile()
 }

+static void _chown(const Path & path, uid_t uid, gid_t gid)
+{
+    checkInterrupt();
+
+    if (lchown(path.c_str(), uid, gid) == -1) {
+	throw SysError(format("change owner and group of `%1%'") % path);
+    }
+    struct stat st = lstat(path);
+    if (S_ISDIR(st.st_mode)) {
+        for (auto & i : readDirectory(path))
+            _chown(path + "/" + i.name, uid, gid);
+    }
+}
+
+
 void DerivationGoal::deleteTmpDir(bool force)
 {
     if (tmpDir != "") {
@@ -2639,6 +2654,12 @@ void DerivationGoal::deleteTmpDir(bool force)
                 format("note: keeping build directory `%2%'")
                 % drvPath % tmpDir);
             chmod(tmpDir.c_str(), 0755);
+            // Change the ownership if clientUid is set. Never change the
+            // ownership or the group to "root" for security reasons.
+            if (settings.clientUid != (uid_t) -1 && settings.clientUid != 0) {
+                _chown(tmpDir, settings.clientUid,
+                       settings.clientGid != 0 ? settings.clientGid : -1);
+            }
         }
         else
             deletePath(tmpDir);
diff --git a/nix/libstore/globals.hh b/nix/libstore/globals.hh
index 8c07e36..7beb1a5 100644
--- a/nix/libstore/globals.hh
+++ b/nix/libstore/globals.hh
@@ -70,6 +70,12 @@ struct Settings {
        subgoal of the same goal) fails. */
     bool keepGoing;
 
+    /* User and groud id of the client issuing the build request.  Used to set
+       the owner and group of the kept temporary directories of failed
+       builds. */
+    uid_t clientUid;
+    gid_t clientGid;
+
     /* Whether, if we cannot realise the known closure corresponding
        to a derivation, we should try to normalise the derivation
        instead. */
diff --git a/nix/nix-daemon/nix-daemon.cc b/nix/nix-daemon/nix-daemon.cc
index 682f9a2..47b67d5 100644
--- a/nix/nix-daemon/nix-daemon.cc
+++ b/nix/nix-daemon/nix-daemon.cc
@@ -960,6 +960,18 @@ static void daemonLoop()
                     strncpy(argvSaved[1], processName.c_str(), strlen(argvSaved[1]));
                 }
 
+#if defined(SO_PEERCRED)
+                /* Store the client's user and group for this connection. This
+                   has to be done in the forked process since it is per
+                   connection. */
+                settings.clientUid = cred.uid;
+                settings.clientGid = cred.gid;
+#else
+                /* Setting these to -1 means: do not change */
+                settings.clientUid = (uid_t) -1;
+                settings.clientGid = (gid_t) -1;
+#endif
+
                 /* Handle the connection. */
                 from.fd = remote;
                 to.fd = remote;
-- 
2.7.4