From c3878f510ec12ca6bf24505989e7463249dab61a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Th=C3=A9ophane=20Hufschmitt?=
 <theophane.hufschmitt@tweag.io>
Date: Tue, 13 Feb 2024 08:28:02 +0100
Subject: [PATCH] Copy the output of fixed-output derivations before
 registering them

It is possible to exfiltrate a file descriptor out of the build sandbox
of FODs, and use it to modify the store path after it has been
registered.
To avoid that issue, don't register the output of the build, but a copy
of it (that will be free of any leaked file descriptor).
---
 src/libstore/build/local-derivation-goal.cc | 6 ++++++
 src/libutil/file-system.cc                  | 5 +++++
 src/libutil/file-system.hh                  | 7 +++++++
 3 files changed, 18 insertions(+)

diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index a9b8de123..d2e2f523e 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/src/libstore/build/local-derivation-goal.cc
@@ -2543,6 +2543,12 @@ SingleDrvOutputs LocalDerivationGoal::registerOutputs()
             [&](const DerivationOutput::CAFixed & dof) {
                 auto & wanted = dof.ca.hash;
 
+                // Replace the output by a fresh copy of itself to make sure
+                // that there's no stale file descriptor pointing to it
+                Path tmpOutput = actualPath + ".tmp";
+                renameFile(actualPath, tmpOutput);
+                copyFile(tmpOutput, actualPath, true);
+
                 auto newInfo0 = newInfoFromCA(DerivationOutput::CAFloating {
                     .method = dof.ca.method,
                     .hashAlgo = wanted.algo,
diff --git a/src/libutil/file-system.cc b/src/libutil/file-system.cc
index b0a3f0797..9dd6a5133 100644
--- a/src/libutil/file-system.cc
+++ b/src/libutil/file-system.cc
@@ -617,6 +617,11 @@ void copy(const fs::directory_entry & from, const fs::path & to, bool andDelete)
     }
 }
 
+void copyFile(const Path & oldPath, const Path & newPath, bool andDelete)
+{
+    return copy(fs::directory_entry(fs::path(oldPath)), fs::path(newPath), andDelete);
+}
+
 void renameFile(const Path & oldName, const Path & newName)
 {
     fs::rename(oldName, newName);
diff --git a/src/libutil/file-system.hh b/src/libutil/file-system.hh
index 464efc242..963265e34 100644
--- a/src/libutil/file-system.hh
+++ b/src/libutil/file-system.hh
@@ -186,6 +186,13 @@ void renameFile(const Path & src, const Path & dst);
  */
 void moveFile(const Path & src, const Path & dst);
 
+/**
+ * Recursively copy the content of `oldPath` to `newPath`. If `andDelete` is
+ * `true`, then also remove `oldPath` (making this equivalent to `moveFile`, but
+ * with the guaranty that the destination will be “fresh”, with no stale inode
+ * or file descriptor pointing to it).
+ */
+void copyFile(const Path & oldPath, const Path & newPath, bool andDelete);
 
 /**
  * Automatic cleanup of resources.