Revert "libstore/build: automatic clean up of unsuccessfully built scratch outputs"

This reverts commit a2189bcb2e as this is the root cause of the critical correctness bug.

Change-Id: Ic85d3d670dc9488d49f03988fe42feb0cf0e7084

doc/manual/rl-next/failed-cve-mitigation.md

@@ -0,0 +1,33 @@
+---
+synopsis: "Revert CVE-2025-52992 failed mitigation"
+issues: [fj#883, fj#887]
+cls: [3420, 3524, 3523, 3522]
+category: "Fixes"
+credits: ["raito", "horrors"]
+---
+
+Following the initial mitigation of **CVE-2025-52992** in `cl/3420`, we
+received reports of **unexpected deletion of in-use store paths**.
+
+Upon investigation, we found that the patch did **not correctly cancel all
+automatic deleters**, resulting in potentially critical path loss during normal
+operation.
+
+Given the severity and time-sensitive nature of the situation ([see incident
+report](https://lix.systems/blog/2025-06-27-lix-critical-bug/)), we evaluated
+possible options to repair the behavior safely. However, we concluded that a
+rushed fix would either:
+
+* **Overdelete**, i.e. breaking running systems, or,
+* **Underdelete**, effectively **reopening CVE-2025-52992** while leaving
+  orphaned paths behind.
+
+As **CVE-2025-52992 has no known exploit vector**, and correctness is critical
+in the Lix project, we have **fully reverted the previous mitigations**.
+
+Moving forward, the Lix team will rework this code path in a **long-term,
+correctness-first fix** on the main branch. We will explore backporting it to
+stable channels once its safety is assured.
+
+We are deeply sorry for the stability incident and the Lix team remain
+available for assisting you in recovering your systems.

src/libstore/build/local-derivation-goal.cc

@@ -363,13 +363,9 @@ void LocalDerivationGoal::cleanupPostOutputsRegisteredModeCheck()
 
 void LocalDerivationGoal::cleanupPostOutputsRegisteredModeNonCheck()
 {
-    /* In the past, redirected outputs were manually tracked for deletion.
+    /* Delete unused redirected outputs (when doing hash rewriting). */
-     * Now that we have the scratch outputs cleaner which are a superset of
+    for (auto & i : redirectedOutputs)
-     * redirected outputs, we just fire all uncancelled automatic deleters now.
+        deletePath(worker.store.Store::toRealPath(i.second));
-     *
-     * This should clean up any paths that IS NOT registered in the database.
-     */
-    scratchOutputsCleaner.clear();
 
     /* Delete the chroot (if we were using one). */
     autoDelChroot.reset(); /* this runs the destructor */
@@ -527,10 +523,6 @@ void LocalDerivationGoal::startBuilder()
                    to use a temporary path */
                 makeFallbackPath(status.known->path);
         scratchOutputs.insert_or_assign(outputName, scratchPath);
-        /* Schedule this scratch output path for automatic deletion
-         * if we do not cancel it, e.g. when registering the outputs.
-         */
-        scratchOutputsCleaner.insert_or_assign(outputName, worker.store.printStorePath(scratchPath));
 
         /* Substitute output placeholders with the scratch output paths.
            We'll use during the build. */
@@ -553,6 +545,8 @@ void LocalDerivationGoal::startBuilder()
             std::string h2 { scratchPath.hashPart() };
             inputRewrites[h1] = h2;
         }
+
+        redirectedOutputs.insert_or_assign(std::move(fixedFinalPath), std::move(scratchPath));
     }
 
     /* Construct the environment passed to the builder. */
@@ -2382,10 +2376,6 @@ SingleDrvOutputs LocalDerivationGoal::registerOutputs()
                 localStore.registerValidPaths({{oldInfo.path, oldInfo}});
             }
 
-            /* Don't register anything, since we already have the
-               previous versions which we're comparing.
-               NOTE: this means that the `.check` path will be automatically deleted.
-            */
             continue;
         }
 
@@ -2409,13 +2399,8 @@ SingleDrvOutputs LocalDerivationGoal::registerOutputs()
         /* If it's a CA path, register it right away. This is necessary if it
            isn't statically known so that we can safely unlock the path before
            the next iteration */
-        if (newInfo.ca) {
+        if (newInfo.ca)
             localStore.registerValidPaths({{newInfo.path, newInfo}});
-            /* Cancel automatic deletion of that output if it was a scratch output. */
-            if (auto cleaner = scratchOutputsCleaner.extract(outputName)) {
-                cleaner.mapped().cancel();
-            }
-        }
 
         infos.emplace(outputName, std::move(newInfo));
     }
@@ -2443,13 +2428,6 @@ SingleDrvOutputs LocalDerivationGoal::registerOutputs()
             infos2.insert_or_assign(newInfo.path, newInfo);
         }
         localStore.registerValidPaths(infos2);
-
-        /* Cancel automatic deletion of that output if it was a scratch output that we just registered. */
-        for (auto & [outputName, _ ] : infos) {
-            if (auto cleaner = scratchOutputsCleaner.extract(outputName)) {
-                cleaner.mapped().cancel();
-            }
-        }
     }
 
     /* In case of a fixed-output derivation hash mismatch, throw an
@@ -2483,13 +2461,6 @@ SingleDrvOutputs LocalDerivationGoal::registerOutputs()
         builtOutputs.emplace(outputName, thisRealisation);
     }
 
-    /* NOTE: At this point, all outputs MAY NOT have been registered.
-     * Therefore, there may remains auto-deleters pending in the cleaner list (`scratchOutputsCleaner`).
-     *
-     * They will be finally deleted but we have no way to assert they all have been, e.g.
-     * `assert(scratchOutputsCleaner.size() == 0)` cannot be written.
-     */
-
     return builtOutputs;
 }
 

src/libstore/build/local-derivation-goal.hh

@@ -107,6 +107,8 @@ struct LocalDerivationGoal : public DerivationGoal
      * Hash rewriting.
      */
     StringMap inputRewrites, outputRewrites;
+    typedef map<StorePath, StorePath> RedirectedOutputs;
+    RedirectedOutputs redirectedOutputs;
 
     /**
      * The outputs paths used during the build.
@@ -123,19 +125,6 @@ struct LocalDerivationGoal : public DerivationGoal
      *   self-references.
      */
     OutputPathMap scratchOutputs;
-    /**
-     * Output paths used during the build are scheduled for
-     * automatic cleanup unless they have been successfully built.
-     *
-     * `registerOutputs` take care of cancelling the cleanups
-     * and clearing this vector.
-     *
-     * `startBuilder` take care of filling this vector
-     * as `scratchOutputs` gets filled.
-     *
-     * This is a map from output names to automatic delete handles.
-     */
-    std::map<std::string, AutoDelete> scratchOutputsCleaner;
 
     /**
      * Path registration info from the previous round, if we're

tests/nixos/default.nix

@@ -146,9 +146,6 @@ in
 
   symlinkResolvconf = runNixOSTestFor "x86_64-linux" ./symlink-resolvconf.nix;
 
-  # Use this test to test things that cannot easily be tested under chroot Nix stores in functional test suite.
-  non-chroot-misc = runNixOSTestFor "x86_64-linux" ./non-chroot-misc;
-
   noNewPrivilegesInSandbox = runNixOSTestFor "x86_64-linux" ./no-new-privileges/sandbox.nix;
 
   noNewPrivilegesOutsideSandbox = runNixOSTestFor "x86_64-linux" ./no-new-privileges/no-sandbox.nix;

tests/nixos/non-chroot-misc/default.nix

@@ -1,34 +0,0 @@
-{ ... }:
-# Misc things we want to test inside of a non redirected, non chroot Nix store.
-let
-  nonAutoCleaningFailingDerivationCode = ''
-    derivation {
-      name = "scratch-failing";
-      system = builtins.currentSystem;
-      builder = "/bin/sh";
-      args = [ (builtins.toFile "builder.sh" "echo bonjour > $out; echo out: $out; false") ];
-    }
-  '';
-in
-{
-  name = "non-chroot-sandbox-misc";
-
-  nodes.machine = {
-  };
-
-  testScript = { nodes }: ''
-    import re
-    start_all()
-
-    # You might ask yourself why write such a convoluted thing?
-    # The condition for fooling Nix into NOT cleaning up the output path are non trivial and unclear.
-    # This is one of those: create a derivation, mkdir or touch the $out path, communicate it back.
-    # Even with a sandboxed Lix, you will observe leftovers before 2.93.0. After this version, this test passes.
-    result = machine.fail("""nix-build --substituters "" -E '${nonAutoCleaningFailingDerivationCode}' 2>&1""")
-    match = re.search(r'out: (\S+)', result)
-    assert match is not None, "Did not find Nix store path in the result of the failing build"
-    outpath = match.group(1).strip()
-    print(f"Found Nix store path: {outpath}")
-    machine.fail(f'stat {outpath}')
-  '';
-}