Index: src/applications/config/option/PhabricatorPHDConfigOptions.php
===================================================================
--- src/applications/config/option/PhabricatorPHDConfigOptions.php
+++ src/applications/config/option/PhabricatorPHDConfigOptions.php
@@ -41,6 +41,14 @@
             "of output, but can help debug issues. Daemons launched in debug ".
             "mode with 'phd debug' are always launched in verbose mode. See ".
             "also 'phd.trace'.")),
+      $this->newOption('phd.user', 'string', null)
+        ->setSummary(pht("System user to run daemons as."))
+        ->setDescription(
+          pht(
+            "Specify a system user to run the daemons as. Primarily, this ".
+            "user will own the working copies of any repositories that ".
+            "Phabricator imports or manages. This option is new and ".
+            "experimental.")),
       $this->newOption('phd.trace', 'bool', false)
         ->setBoolOptions(
           array(
Index: src/applications/diffusion/controller/DiffusionServeController.php
===================================================================
--- src/applications/diffusion/controller/DiffusionServeController.php
+++ src/applications/diffusion/controller/DiffusionServeController.php
@@ -326,7 +326,10 @@
 
     $input = PhabricatorStartup::getRawInput();
 
-    list($err, $stdout, $stderr) = id(new ExecFuture('%s', $bin))
+    $command = csprintf('%s', $bin);
+    $command = PhabricatorDaemon::sudoCommandAsDaemonUser($command);
+
+    list($err, $stdout, $stderr) = id(new ExecFuture('%C', $command))
       ->setEnv($env, true)
       ->write($input)
       ->resolve();
@@ -422,7 +425,10 @@
       $input = strlen($input)."\n".$input."0\n";
     }
 
-    list($err, $stdout, $stderr) = id(new ExecFuture('%s serve --stdio', $bin))
+    $command = csprintf('%s serve --stdio', $bin);
+    $command = PhabricatorDaemon::sudoCommandAsDaemonUser($command);
+
+    list($err, $stdout, $stderr) = id(new ExecFuture('%C', $command))
       ->setEnv($env, true)
       ->setCWD($repository->getLocalPath())
       ->write("{$cmd}\n{$args}{$input}")
@@ -545,5 +551,6 @@
 
     return $has_pack && $is_hangup;
   }
+
 }
 
Index: src/applications/diffusion/ssh/DiffusionSSHGitReceivePackWorkflow.php
===================================================================
--- src/applications/diffusion/ssh/DiffusionSSHGitReceivePackWorkflow.php
+++ src/applications/diffusion/ssh/DiffusionSSHGitReceivePackWorkflow.php
@@ -22,9 +22,10 @@
     // This is a write, and must have write access.
     $this->requireWriteAccess();
 
-    $future = new ExecFuture(
-      'git-receive-pack %s',
-      $repository->getLocalPath());
+    $command = csprintf('git-receive-pack %s', $repository->getLocalPath());
+    $command = PhabricatorDaemon::sudoCommandAsDaemonUser($command);
+
+    $future = new ExecFuture('%C', $command);
 
     $err = $this->newPassthruCommand()
       ->setIOChannel($this->getIOChannel())
Index: src/applications/diffusion/ssh/DiffusionSSHGitUploadPackWorkflow.php
===================================================================
--- src/applications/diffusion/ssh/DiffusionSSHGitUploadPackWorkflow.php
+++ src/applications/diffusion/ssh/DiffusionSSHGitUploadPackWorkflow.php
@@ -19,7 +19,10 @@
     $path = head($args->getArg('dir'));
     $repository = $this->loadRepository($path);
 
-    $future = new ExecFuture('git-upload-pack %s', $repository->getLocalPath());
+    $command = csprintf('git-upload-pack -- %s', $repository->getLocalPath());
+    $command = PhabricatorDaemon::sudoCommandAsDaemonUser($command);
+
+    $future = new ExecFuture('%C', $command);
 
     $err = $this->newPassthruCommand()
       ->setIOChannel($this->getIOChannel())
Index: src/applications/diffusion/ssh/DiffusionSSHMercurialServeWorkflow.php
===================================================================
--- src/applications/diffusion/ssh/DiffusionSSHMercurialServeWorkflow.php
+++ src/applications/diffusion/ssh/DiffusionSSHMercurialServeWorkflow.php
@@ -39,12 +39,12 @@
       throw new Exception("Expected `hg ... serve`!");
     }
 
-    $future = new ExecFuture(
-      'hg -R %s serve --stdio',
-      $repository->getLocalPath());
+    $command = csprintf('hg -R %s serve --stdio', $repository->getLocalPath());
+    $command = PhabricatorDaemon::sudoCommandAsDaemonUser($command);
 
-    $io_channel = $this->getIOChannel();
+    $future = new ExecFuture('%C', $command);
 
+    $io_channel = $this->getIOChannel();
     $protocol_channel = new DiffusionSSHMercurialWireClientProtocolChannel(
       $io_channel);
 
Index: src/applications/diffusion/ssh/DiffusionSSHSubversionServeWorkflow.php
===================================================================
--- src/applications/diffusion/ssh/DiffusionSSHSubversionServeWorkflow.php
+++ src/applications/diffusion/ssh/DiffusionSSHSubversionServeWorkflow.php
@@ -38,7 +38,10 @@
       throw new Exception("Expected `svnserve -t`!");
     }
 
-    $future = new ExecFuture('svnserve -t');
+    $command = csprintf('svnserve -t');
+    $command = PhabricatorDaemon::sudoCommandAsDaemonUser($command);
+
+    $future = new ExecFuture('%C', $command);
 
     $this->inProtocol = new DiffusionSubversionWireProtocol();
     $this->outProtocol = new DiffusionSubversionWireProtocol();
Index: src/applications/diffusion/ssh/DiffusionSSHWorkflow.php
===================================================================
--- src/applications/diffusion/ssh/DiffusionSSHWorkflow.php
+++ src/applications/diffusion/ssh/DiffusionSSHWorkflow.php
@@ -120,5 +120,4 @@
     return $this->hasWriteAccess;
   }
 
-
 }
Index: src/infrastructure/daemon/PhabricatorDaemon.php
===================================================================
--- src/infrastructure/daemon/PhabricatorDaemon.php
+++ src/infrastructure/daemon/PhabricatorDaemon.php
@@ -19,4 +19,35 @@
     return PhabricatorUser::getOmnipotentUser();
   }
 
+
+  /**
+   * Format a command so it executes as the daemon user, if a daemon user is
+   * defined. This wraps the provided command in `sudo -u ...`, roughly.
+   *
+   * @param   PhutilCommandString Command to execute.
+   * @return  PhutilCommandString `sudo` version of the command.
+   */
+  public static function sudoCommandAsDaemonUser($command) {
+    $user = PhabricatorEnv::getEnvConfig('phd.user');
+    if (!$user) {
+      // No daemon user is set, so just run this as ourselves.
+      return $command;
+    }
+
+    // Get the absolute path so we're safe against the caller wiping out
+    // PATH.
+    $sudo = Filesystem::resolveBinary('sudo');
+    if (!$sudo) {
+      throw new Exception(pht("Unable to find 'sudo'!"));
+    }
+
+    // Flags here are:
+    //
+    //   -E: Preserve the environment.
+    //   -n: Non-interactive. Exit with an error instead of prompting.
+    //   -u: Which user to sudo to.
+
+    return csprintf('%s -E -n -u %s -- %C', $sudo, $user, $command);
+  }
+
 }