Fix passing custom CA files into the builtin:fetchurl sandbox

Without this, verifying TLS certificates would fail on macOS, as well
as any system that doesn't have a certificate file at /etc/ssl/certs/ca-certificates.crt,
which includes e.g. Fedora.

Change-Id: Iaa2e0e9db3747645b5482c82e3e0e4e8f229f5f9

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

@@ -1361,13 +1361,20 @@ void LocalDerivationGoal::runChild()
 
         bool setUser = true;
 
-        /* Make the contents of netrc available to builtin:fetchurl
+        /* Make the contents of netrc and the CA certificate bundle
-           (which may run under a different uid and/or in a sandbox). */
+           available to builtin:fetchurl (which may run under a
+           different uid and/or in a sandbox). */
         std::string netrcData;
-        try {
+        std::string caFileData;
-            if (drv->isBuiltin() && drv->builder == "builtin:fetchurl" && !derivationType->isSandboxed())
+        if (drv->isBuiltin() && drv->builder == "builtin:fetchurl" && !derivationType->isSandboxed()) {
+            try {
                 netrcData = readFile(settings.netrcFile);
-        } catch (SysError &) { }
+            } catch (SysError &) { }
+
+            try {
+                caFileData = readFile(settings.caFile);
+            } catch (SysError &) { }
+        }
 
 #if __linux__
         if (useChroot) {
@@ -1802,7 +1809,7 @@ 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 +6,7 @@
 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

@@ -7,7 +7,7 @@
 
 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
@@ -17,6 +17,9 @@ 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);

tests/nixos/fetchurl.nix

@@ -67,6 +67,9 @@ in
     out = machine.succeed("curl https://good/index.html")
     assert out == "hello world\n"
 
+    out = machine.succeed("cat ${badCert}/cert.pem > /tmp/cafile.pem; curl --cacert /tmp/cafile.pem https://bad/index.html")
+    assert out == "foobar\n"
+
     # Fetching from a server with a trusted cert should work.
     machine.succeed("nix build --no-substitute --expr 'import <nix/fetchurl.nix> { url = \"https://good/index.html\"; hash = \"sha256-qUiQTy8PR5uPgZdpSzAYSw0u0cHNKh7A+4XSmaGSpEc=\"; }'")
 
@@ -74,5 +77,8 @@ in
     err = machine.fail("nix build --no-substitute --expr 'import <nix/fetchurl.nix> { url = \"https://bad/index.html\"; hash = \"sha256-rsBwZF/lPuOzdjBZN2E08FjMM3JHyXit0Xi2zN+wAZ8=\"; }' 2>&1")
     print(err)
     assert "SSL certificate problem: self-signed certificate" in err or "SSL peer certificate or SSH remote key was not OK" in err
+
+    # Fetching from a server with a trusted cert should work via environment variable override.
+    machine.succeed("NIX_SSL_CERT_FILE=/tmp/cafile.pem nix build --no-substitute --expr 'import <nix/fetchurl.nix> { url = \"https://bad/index.html\"; hash = \"sha256-rsBwZF/lPuOzdjBZN2E08FjMM3JHyXit0Xi2zN+wAZ8=\"; }'")
   '';
 }