libexpr/flake: allow automatic rejection of configuration options from flakes

The `allow-flake-configuration` option allows the user to control whether to
accept configuration options supplied by flakes. Unfortunately, setting this
to false really meant "ask each time" (with an option to remember the choice
for each specific option encountered). Let no mean no, and introduce (and
default to) a separate value for the "ask each time" behaviour.

Co-Authored-By: Jade Lovelace <lix@jade.fyi>
Change-Id: I7ccd67a95bfc92cffc1ebdc972d243f5191cc1b4

6 files changed, 106 insertions, 25 deletions

diff --git a/doc/manual/rl-next/reject-flake-config.md b/doc/manual/rl-next/reject-flake-config.md
new file mode 100644
index 000000000..d38edc80c
--- /dev/null
+++ b/doc/manual/rl-next/reject-flake-config.md
@@ -0,0 +1,9 @@
+---
+synopsis: Allow automatic rejection of configuration options from flakes
+cls: [1541]
+credits: [alois31]
+category: Improvements
+---
+
+Setting `accept-flake-config` to `false` now respects user choice by automatically rejecting configuration options set by flakes.
+The old behaviour of asking each time is still available (and default) by setting it to the special value `ask`.
diff --git a/src/libexpr/flake/config.cc b/src/libexpr/flake/config.cc
index b330d96f9..adcf7fd10 100644
--- a/src/libexpr/flake/config.cc
+++ b/src/libexpr/flake/config.cc
@@ -51,30 +51,46 @@ void ConfigFile::apply()
         else
             assert(false);
 
-        if (!whitelist.count(baseName) && !nix::fetchSettings.acceptFlakeConfig) {
-            bool trusted = false;
-            auto trustedList = readTrustedList();
-            auto tlname = get(trustedList, name);
-            if (auto saved = tlname ? get(*tlname, valueS) : nullptr) {
-                trusted = *saved;
-                printInfo("Using saved setting for '%s = %s' from ~/.local/share/nix/trusted-settings.json.", name, valueS);
-            } else {
-                // FIXME: filter ANSI escapes, newlines, \r, etc.
-                if (std::tolower(logger->ask(fmt("do you want to allow configuration setting '%s' to be set to '" ANSI_RED "%s" ANSI_NORMAL "' (y/N)?", name, valueS)).value_or('n')) == 'y') {
-                    trusted = true;
-                }
-                if (std::tolower(logger->ask(fmt("do you want to permanently mark this value as %s (y/N)?",  trusted ? "trusted": "untrusted" )).value_or('n')) == 'y') {
-                    trustedList[name][valueS] = trusted;
-                    writeTrustedList(trustedList);
+        bool trusted = whitelist.count(baseName);
+        if (!trusted) {
+            switch (nix::fetchSettings.acceptFlakeConfig) {
+            case AcceptFlakeConfig::True: {
+                trusted = true;
+                break;
+            }
+            case AcceptFlakeConfig::Ask: {
+                auto trustedList = readTrustedList();
+                auto tlname = get(trustedList, name);
+                if (auto saved = tlname ? get(*tlname, valueS) : nullptr) {
+                    trusted = *saved;
+                    printInfo("Using saved setting for '%s = %s' from ~/.local/share/nix/trusted-settings.json.", name, valueS);
+                } else {
+                    // FIXME: filter ANSI escapes, newlines, \r, etc.
+                    if (std::tolower(logger->ask(fmt("Do you want to allow configuration setting '%s' to be set to '" ANSI_RED "%s" ANSI_NORMAL "' (y/N)? This may allow the flake to gain root, see the nix.conf manual page.", name, valueS)).value_or('n')) == 'y') {
+                        trusted = true;
+                    } else {
+                        warn("you can set '%s' to '%b' to automatically reject configuration options supplied by flakes", "accept-flake-config", false);
+                    }
+                    if (std::tolower(logger->ask(fmt("do you want to permanently mark this value as %s (y/N)?",  trusted ? "trusted": "untrusted" )).value_or('n')) == 'y') {
+                        trustedList[name][valueS] = trusted;
+                        writeTrustedList(trustedList);
+                    }
                 }
+                break;
             }
-            if (!trusted) {
-                warn("ignoring untrusted flake configuration setting '%s'.\nPass '%s' to trust it", name, "--accept-flake-config");
-                continue;
+            case nix::AcceptFlakeConfig::False: {
+                trusted = false;
+                break;
+            };
             }
         }
 
-        globalConfig.set(name, valueS);
+        if (trusted) {
+            debug("accepting trusted flake configuration setting '%s'", name);
+            globalConfig.set(name, valueS);
+        } else {
+            warn("ignoring untrusted flake configuration setting '%s', pass '%s' to trust it (may allow the flake to gain root, see the nix.conf manual page)", name, "--accept-flake-config");
+        }
     }
 }
 
diff --git a/src/libfetchers/fetch-settings.cc b/src/libfetchers/fetch-settings.cc
index e7d5244dc..aeb3c542b 100644
--- a/src/libfetchers/fetch-settings.cc
+++ b/src/libfetchers/fetch-settings.cc
@@ -1,7 +1,50 @@
+#include "abstract-setting-to-json.hh"
+#include "args.hh"
+#include "config-impl.hh"
 #include "fetch-settings.hh"
 
+#include <nlohmann/json.hpp>
+
 namespace nix {
 
+template<> AcceptFlakeConfig BaseSetting<AcceptFlakeConfig>::parse(const std::string & str) const
+{
+    if (str == "true") return AcceptFlakeConfig::True;
+    else if (str == "ask") return AcceptFlakeConfig::Ask;
+    else if (str == "false") return AcceptFlakeConfig::False;
+    else throw UsageError("option '%s' has invalid value '%s'", name, str);
+}
+
+template<> std::string BaseSetting<AcceptFlakeConfig>::to_string() const
+{
+    if (value == AcceptFlakeConfig::True) return "true";
+    else if (value == AcceptFlakeConfig::Ask) return "ask";
+    else if (value == AcceptFlakeConfig::False) return "false";
+    else abort();
+}
+
+template<> void BaseSetting<AcceptFlakeConfig>::convertToArg(Args & args, const std::string & category)
+{
+    args.addFlag({
+        .longName = name,
+        .description = "Accept Lix configuration options from flakes without confirmation. This allows flakes to gain root access to your machine if you are a trusted user; see the nix.conf manual page for more details.",
+        .category = category,
+        .handler = {[this]() { override(AcceptFlakeConfig::True); }}
+    });
+    args.addFlag({
+        .longName = "ask-" + name,
+        .description = "Ask whether to accept Lix configuration options from flakes.",
+        .category = category,
+        .handler = {[this]() { override(AcceptFlakeConfig::Ask); }}
+    });
+    args.addFlag({
+        .longName = "no-" + name,
+        .description = "Reject Lix configuration options from flakes.",
+        .category = category,
+        .handler = {[this]() { override(AcceptFlakeConfig::False); }}
+    });
+}
+
 FetchSettings::FetchSettings()
 {
 }
diff --git a/src/libfetchers/fetch-settings.hh b/src/libfetchers/fetch-settings.hh
index 6fb260c3a..93123463c 100644
--- a/src/libfetchers/fetch-settings.hh
+++ b/src/libfetchers/fetch-settings.hh
@@ -11,6 +11,8 @@
 
 namespace nix {
 
+enum class AcceptFlakeConfig { False, Ask, True };
+
 struct FetchSettings : public Config
 {
     FetchSettings();
@@ -86,15 +88,21 @@ struct FetchSettings : public Config
         "Whether to use flake registries to resolve flake references.",
         {}, true, Xp::Flakes};
 
-    Setting<bool> acceptFlakeConfig{this, false, "accept-flake-config",
+    Setting<AcceptFlakeConfig> acceptFlakeConfig{
+        this, AcceptFlakeConfig::Ask, "accept-flake-config",
         R"(
           Whether to accept Lix configuration from the `nixConfig` attribute of
-          a flake without prompting. This is almost always a very bad idea.
-
-          Setting this setting as a trusted user allows Nix flakes to gain root
+          a flake. Doing so as a trusted user allows Nix flakes to gain root
           access on your machine if they set one of the several
           trusted-user-only settings that execute commands as root.
 
+          If set to `true`, such configuration will be accepted without asking;
+          this is almost always a very bad idea. Setting this to `ask` will
+          prompt the user each time whether to allow a certain configuration
+          option set this way, and offer to optionally remember their choice.
+          When set to `false`, the configuration will be automatically
+          declined.
+
           See [multi-user installations](@docroot@/installation/multi-user.md)
           for more details on the Lix security model.
         )",
diff --git a/tests/functional/experimental-features.sh b/tests/functional/experimental-features.sh
index 9ee4a53d4..658ae3cc4 100644
--- a/tests/functional/experimental-features.sh
+++ b/tests/functional/experimental-features.sh
@@ -32,7 +32,7 @@ NIX_CONFIG='
   experimental-features = nix-command
   accept-flake-config = true
 ' nix config show accept-flake-config 1>$TEST_ROOT/stdout 2>$TEST_ROOT/stderr
-grepQuiet "false" $TEST_ROOT/stdout
+grepQuiet "ask" $TEST_ROOT/stdout
 grepQuiet "Ignoring setting 'accept-flake-config' because experimental feature 'flakes' is not enabled" $TEST_ROOT/stderr
 
 # 'flakes' experimental-feature is disabled after, ignore and warn
@@ -40,7 +40,7 @@ NIX_CONFIG='
   accept-flake-config = true
   experimental-features = nix-command
 ' nix config show accept-flake-config 1>$TEST_ROOT/stdout 2>$TEST_ROOT/stderr
-grepQuiet "false" $TEST_ROOT/stdout
+grepQuiet "ask" $TEST_ROOT/stdout
 grepQuiet "Ignoring setting 'accept-flake-config' because experimental feature 'flakes' is not enabled" $TEST_ROOT/stderr
 
 # 'flakes' experimental-feature is enabled before, process
diff --git a/tests/functional/flakes/config.sh b/tests/functional/flakes/config.sh
index d1941a6be..f7eae06d8 100644
--- a/tests/functional/flakes/config.sh
+++ b/tests/functional/flakes/config.sh
@@ -28,6 +28,11 @@ nix build < /dev/null
 (! [[ -f post-hook-ran ]])
 clearStore
 
+# likewise with no-accept-flake-config
+nix build --no-accept-flake-config
+(! [[ -f post-hook-ran ]])
+clearStore
+
 nix build --accept-flake-config
 test -f post-hook-ran || fail "The post hook should have ran"