Security: remove support for $(...) in config keys with [$e] marker.

Summary:
It is very unclear at this point what a valid use case for this feature
would possibly be. The old documentation only mentions $(hostname) as
an example, which can be done with $HOSTNAME instead.

Note that $(...) is still supported in Exec lines of desktop files,
this does not require [$e] anyway (and actually works better without it,
otherwise the $ signs need to be doubled to obey kconfig $e escaping rules...).

Test Plan:
ctest passes; various testcases with $(...) in desktop files,
directory files, and config files, no longer execute commands.

Reviewers: mdawson, aacid, broulik, davidedmundson, kossebau, apol, sitter, security-team

Reviewed By: mdawson, davidedmundson

Subscribers: ZaWertun, rikmills, fvogt, ngraham, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D22979

3 files changed, 7 insertions, 51 deletions

diff --git a/autotests/kconfigtest.cpp b/autotests/kconfigtest.cpp
index 410b5b80..9af3b464 100644
--- a/autotests/kconfigtest.cpp
+++ b/autotests/kconfigtest.cpp
@@ -38,7 +38,7 @@
 #include <utime.h>
 #endif
 #ifndef Q_OS_WIN
-#include <unistd.h> // gethostname
+#include <unistd.h> // getuid
 #endif
 
 KCONFIGGROUP_DECLARE_ENUM_QOBJECT(KConfigTest, Testing)
@@ -546,14 +546,8 @@ void KConfigTest::testPath()
     QCOMPARE(group.readPathEntry("withBraces", QString()), QString("file://" + HOMEPATH));
     QVERIFY(group.hasKey("URL"));
     QCOMPARE(group.readEntry("URL", QString()), QString("file://" + HOMEPATH));
-#if !defined(Q_OS_WIN32) && !defined(Q_OS_MAC)
-    // I don't know if this will work on windows
-    // This test hangs on OS X
     QVERIFY(group.hasKey("hostname"));
-    char hostname[256];
-    QVERIFY(::gethostname(hostname, sizeof(hostname)) == 0);
-    QCOMPARE(group.readEntry("hostname", QString()), QString::fromLatin1(hostname));
-#endif
+    QCOMPARE(group.readEntry("hostname", QString()), QStringLiteral("(hostname)")); // the $ got removed because empty var name
     QVERIFY(group.hasKey("noeol"));
     QCOMPARE(group.readEntry("noeol", QString()), QString("foo"));
 
diff --git a/docs/options.md b/docs/options.md
index c634c000..4a6e9bc1 100644
--- a/docs/options.md
+++ b/docs/options.md
@@ -67,18 +67,15 @@ environment variables (and `XDG_CONFIG_HOME` in particular).
 Shell Expansion
 ---------------
 
-If an entry is marked with `$e`, environment variables and shell commands will
-be expanded.
+If an entry is marked with `$e`, environment variables will be expanded.
 
     Name[$e]=$USER
-    Host[$e]=$(hostname)
 
 When the "Name" entry is read `$USER` will be replaced with the value of the
-`$USER` environment variable, and `$(hostname)` will be replaced with the output
-of the `hostname` command.
+`$USER` environment variable.
 
-Note that the application will replace `$USER` and `$(hostname)` with their
-respective expanded values after saving. To prevent this combine the `$e` option
+Note that the application will replace `$USER` with its
+expanded value after saving. To prevent this combine the `$e` option
 with `$i` (immmutable) option.  For example:
 
     Name[$ei]=$USER
diff --git a/src/core/kconfig.cpp b/src/core/kconfig.cpp
index e1b11edd..f6824ce6 100644
--- a/src/core/kconfig.cpp
+++ b/src/core/kconfig.cpp
@@ -28,19 +28,6 @@
 #include <cstdlib>
 #include <fcntl.h>
 
-#ifdef _MSC_VER
-static inline FILE *popen(const char *cmd, const char *mode)
-{
-    return _popen(cmd, mode);
-}
-static inline int pclose(FILE *stream)
-{
-    return _pclose(stream);
-}
-#else
-#include <unistd.h>
-#endif
-
 #include "kconfigbackend_p.h"
 #include "kconfiggroup.h"
 
@@ -183,29 +170,7 @@ QString KConfigPrivate::expandString(const QString &value)
     int nDollarPos = aValue.indexOf(QLatin1Char('$'));
     while (nDollarPos != -1 && nDollarPos + 1 < aValue.length()) {
         // there is at least one $
-        if (aValue[nDollarPos + 1] == QLatin1Char('(')) {
-            int nEndPos = nDollarPos + 1;
-            // the next character is not $
-            while ((nEndPos <= aValue.length()) && (aValue[nEndPos] != QLatin1Char(')'))) {
-                nEndPos++;
-            }
-            nEndPos++;
-            QString cmd = aValue.mid(nDollarPos + 2, nEndPos - nDollarPos - 3);
-
-            QString result;
-
-// FIXME: wince does not have pipes
-#ifndef _WIN32_WCE
-            FILE *fs = popen(QFile::encodeName(cmd).data(), "r");
-            if (fs) {
-                QTextStream ts(fs, QIODevice::ReadOnly);
-                result = ts.readAll().trimmed();
-                pclose(fs);
-            }
-#endif
-            aValue.replace(nDollarPos, nEndPos - nDollarPos, result);
-            nDollarPos += result.length();
-        } else if (aValue[nDollarPos + 1] != QLatin1Char('$')) {
+        if (aValue[nDollarPos + 1] != QLatin1Char('$')) {
             int nEndPos = nDollarPos + 1;
             // the next character is not $
             QStringRef aVarName;