fix passing CA files into builtins:fetchurl sandbox

This patch has been manually adapted from
14dc84ed03

Tested with:

$ NIX_SSL_CERT_FILE=$(nix-build '<nixpkgs>' -A cacert)/etc/ssl/certs/ca-bundle.crt nix-build --store $(mktemp -d) -E 'import <nix/fetchurl.nix> { url = https://google.com; }'
warning: found empty hash, assuming 'sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA='
this derivation will be built:
  /nix/store/4qljhy0jj2b0abjzpsbyarpia1bqylwc-google.com.drv
building '/nix/store/4qljhy0jj2b0abjzpsbyarpia1bqylwc-google.com.drv'...
error:
       … writing file '/nix/store/0zynn4n8yx59bczy1mgh1lq2rnprvvrc-google.com'

       error: unable to download 'https://google.com': Problem with the SSL CA cert (path? access rights?) (77)
error: builder for '/nix/store/4qljhy0jj2b0abjzpsbyarpia1bqylwc-google.com.drv' failed with exit code 1

Now returns:

nix-env % NIX_SSL_CERT_FILE=$(nix-build '<nixpkgs>' -A cacert)/etc/ssl/certs/ca-bundle.crt nix-build --store $(mktemp -d) -E 'import <nix/fetchurl.nix> { url = https://google.com; }'
this derivation will be built:
  /nix/store/4qljhy0jj2b0abjzpsbyarpia1bqylwc-google.com.drv
building '/nix/store/4qljhy0jj2b0abjzpsbyarpia1bqylwc-google.com.drv'...
error: hash mismatch in fixed-output derivation '/nix/store/4qljhy0jj2b0abjzpsbyarpia1bqylwc-google.com.drv':
         specified: sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
            got:    sha256-5xXEhGtnRdopaUTqaz2M1o2NE7ovhU0SjcSOPwntqwY=

(cherry picked from commit 1fbdf409524bb350b8614f3d95067cb9ba3c57f2)

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

@@ -1740,13 +1740,20 @@ void LocalDerivationGoal::runChild()
 
         bool setUser = true;
 
-        /* Make the contents of netrc available to builtin:fetchurl
-           (which may run under a different uid and/or in a sandbox). */
+        /* Make the contents of netrc and the CA certificate bundle
+           available to builtin:fetchurl (which may run under a
+           different uid and/or in a sandbox). */
         std::string netrcData;
-        try {
-            if (drv->isBuiltin() && drv->builder == "builtin:fetchurl")
-                netrcData = readFile(settings.netrcFile);
-        } catch (SysError &) { }
+        std::string caFileData;
+        if (drv->isBuiltin() && drv->builder == "builtin:fetchurl") {
+           try {
+               netrcData = readFile(settings.netrcFile);
+           } catch (SysError &) { }
+
+           try {
+               caFileData = readFile(settings.caFile);
+           } catch (SysError &) { }
+        }
 
 #if __linux__
         if (useChroot) {
@@ -2174,7 +2181,7 @@ void LocalDerivationGoal::runChild()
                     e.second = rewriteStrings(e.second, inputRewrites);
 
                 if (drv->builder == "builtin:fetchurl")
-                    builtinFetchurl(drv2, netrcData);
+                    builtinFetchurl(drv2, netrcData, caFileData);
                 else if (drv->builder == "builtin:buildenv")
                     builtinBuildenv(drv2);
                 else if (drv->builder == "builtin:unpack-channel")

src/libstore/builtins.hh

@@ -6,7 +6,9 @@
 namespace nix {
 
 // TODO: make pluggable.
-void builtinFetchurl(const BasicDerivation & drv, const std::string & netrcData);
+void builtinFetchurl(const BasicDerivation & drv,
+    const std::string & netrcData,
+    const std::string & caFileData);
 void builtinUnpackChannel(const BasicDerivation & drv);
 
 }

src/libstore/builtins/fetchurl.cc

@@ -6,7 +6,10 @@
 
 namespace nix {
 
-void builtinFetchurl(const BasicDerivation & drv, const std::string & netrcData)
+void builtinFetchurl(
+    const BasicDerivation & drv,
+    const std::string & netrcData,
+    const std::string & caFileData)
 {
     /* Make the host's netrc data available. Too bad curl requires
        this to be stored in a file. It would be nice if we could just
@@ -16,6 +19,9 @@ void builtinFetchurl(const BasicDerivation & drv, const std::string & netrcData)
         writeFile(settings.netrcFile, netrcData, 0600);
     }
 
+    settings.caFile = "ca-certificates.crt";
+    writeFile(settings.caFile, caFileData, 0600);
+
     auto getAttr = [&](const std::string & name) {
         auto i = drv.env.find(name);
         if (i == drv.env.end()) throw Error("attribute '%s' missing", name);