package/mbedtls: make compression support a config option

Enabling TLS compression may make mbedTLS vulnerable to the
CRIME attack [1]. It should not be enabled unless is is sure CRIME and
similar attacks are not applicable to the particulare situation.

As zlib is probably enabled in most systems, the user might end up with
a vulnerable system without knowing. So, instead of enabling compression
support if the zlib package is available, we make the compression support
a config option. This way, the user has to explicitly enable compression
support and is warned by the help text about the risk.

[1] https://tls.mbed.org/kb/how-to/deflate-compression-in-ssl-tls

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/mbedtls/Config.in

@@ -17,4 +17,16 @@ config BR2_PACKAGE_MBEDTLS_PROGRAMS
 	  This option enables the installation and the build of
 	  mbed TLS companion programs.
 
+config BR2_PACKAGE_MBEDTLS_COMPRESSION
+	bool "enable compression support"
+	select BR2_PACKAGE_ZLIB
+	help
+	  Enable support for compression of the content data before it
+	  enters the secure channel as described in RFC 3749.
+	  
+	  Warning: TLS compression may make you vulnerable to the CRIME
+	  attack. You should not enable it unless you know for sure CRIME
+	  and similar attacks are not applicable to your particular
+	  situation.
+
 endif

package/mbedtls/mbedtls.mk

@@ -39,7 +39,7 @@ MBEDTLS_CONF_OPTS += \
 	-DUSE_SHARED_MBEDTLS_LIBRARY=ON -DUSE_STATIC_MBEDTLS_LIBRARY=OFF
 endif
 
-ifeq ($(BR2_PACKAGE_ZLIB),y)
+ifeq ($(BR2_PACKAGE_MBEDTLS_COMPRESSION),y)
 MBEDTLS_CONF_OPTS += -DENABLE_ZLIB_SUPPORT=ON
 MBEDTLS_DEPENDENCIES += zlib
 define MBEDTLS_ENABLE_ZLIB