[Buildroot] [PATCH 4/5] dropbear: add option to disable CBC mode ciphers

[Baruch Siach <baruch@tkos.co.il>]

Hi Thomas,

On Wed, Jul 04, 2018 at 09:07:38AM +0200, Thomas De Schampheleire wrote:
> CBC mode ciphers are considered insecure. Add an option to disable it.

We have a patch from Stefan S?rensen to disable all weak algorithms by 
default:

  http://patchwork.ozlabs.org/patch/938595/

I prefer Stefan's patch over this and the next patch.

Stefan's patch doesn't deal with the MD5 HMAC as the next patch does. But I 
find it highly unlikely that upstream would enable MD5 by default ever again. 
Upstream commit 34ee32607598 adds this code in sysoptions.h:

/* might be needed for compatibility with very old implementations */
#ifndef DROPBEAR_MD5_HMAC
#define DROPBEAR_MD5_HMAC 0
#endif

default_options.h doesn't mention MD5 at all.

baruch

> Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
> ---
>  package/dropbear/Config.in   | 7 +++++++
>  package/dropbear/dropbear.mk | 8 ++++++++
>  2 files changed, 15 insertions(+)
> 
> diff --git a/package/dropbear/Config.in b/package/dropbear/Config.in
> index 5d6b83b6d1..d92420ac81 100644
> --- a/package/dropbear/Config.in
> +++ b/package/dropbear/Config.in
> @@ -35,6 +35,13 @@ config BR2_PACKAGE_DROPBEAR_DISABLE_REVERSEDNS
>  	  on systems without working DNS, as connections otherwise
>  	  stall until DNS times out.
>  
> +config BR2_PACKAGE_DROPBEAR_DISABLE_CBC
> +	bool "disable CBC mode ciphers"
> +	help
> +	  Cipher Block Chaining (CBC) may allow an attacker to recover
> +	  plaintext messages from the ciphertext. For higher security, it is
> +	  recommended to disable it (and thus enable this option).
> +
>  config BR2_PACKAGE_DROPBEAR_SMALL
>  	bool "optimize for size"
>  	default y
> diff --git a/package/dropbear/dropbear.mk b/package/dropbear/dropbear.mk
> index bb902bc7ce..dc233aab53 100644
> --- a/package/dropbear/dropbear.mk
> +++ b/package/dropbear/dropbear.mk
> @@ -71,6 +71,10 @@ define DROPBEAR_DISABLE_STANDALONE
>  	echo '#define NON_INETD_MODE 0'                 >> $(@D)/localoptions.h
>  endef
>  
> +define DROPBEAR_DISABLE_CBC_CIPHERS
> +	echo '#define DROPBEAR_ENABLE_CBC_MODE 0'       >> $(@D)/localoptions.h
> +endef
> +
>  define DROPBEAR_INSTALL_INIT_SYSTEMD
>  	$(INSTALL) -D -m 644 package/dropbear/dropbear.service \
>  		$(TARGET_DIR)/usr/lib/systemd/system/dropbear.service
> @@ -92,6 +96,10 @@ ifeq ($(BR2_PACKAGE_DROPBEAR_DISABLE_REVERSEDNS),)
>  DROPBEAR_POST_EXTRACT_HOOKS += DROPBEAR_ENABLE_REVERSE_DNS
>  endif
>  
> +ifeq ($(BR2_PACKAGE_DROPBEAR_DISABLE_CBC),y)
> +DROPBEAR_POST_EXTRACT_HOOKS += DROPBEAR_DISABLE_CBC_CIPHERS
> +endif
> +
>  ifeq ($(BR2_PACKAGE_DROPBEAR_SMALL),y)
>  DROPBEAR_CONF_OPTS += --disable-zlib --enable-bundled-libtom
>  else
> -- 
> 2.16.4
> 

-- 
     http://baruch.siach.name/blog/                  ~. .~   Tk Open Systems
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   - baruch at tkos.co.il - tel: +972.2.679.5364, http://www.tkos.co.il -