Re: [PATCH] x86, vsyscall: add CONFIG to control default

[Josh Triplett <josh@joshtriplett.org>]

On Wed, Aug 12, 2015 at 05:55:19PM -0700, Kees Cook wrote:
> Most modern systems can run with vsyscall=none. In an effort to provide
> a way for build-time defaults to lack legacy settings, this adds a new
> CONFIG to select the type of vsyscall mapping to use, similar to the
> existing "vsyscall" command line parameter.
> 
> Signed-off-by: Kees Cook <keescook@chromium.org>

Seems reasonable to me.  One question, though: is there *any* reason to
choose "native" over "emulate"?  (Does "emulate" have a sufficient
performance penalty to matter, and do people running old glibc really
care about that performance while still not wanting to upgrade?)
If there is a reason, could you please document it in the
descriptions of the "native" and "emulate" options (as an upside and a
downside, respectively)?  If there isn't, you might consider a patch to
remove "native".

>  arch/x86/Kconfig                      | 49 +++++++++++++++++++++++++++++++++++
>  arch/x86/entry/vsyscall/vsyscall_64.c |  9 ++++++-
>  2 files changed, 57 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index b3a1a5d77d92..fbd0fad714a1 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -2010,6 +2010,55 @@ config COMPAT_VDSO
>  	  If unsure, say N: if you are compiling your own kernel, you
>  	  are unlikely to be using a buggy version of glibc.
>  
> +choice
> +	prompt "vsyscall table for legacy applications"
> +	depends on X86_64
> +	default LEGACY_VSYSCALL_EMULATE
> +	help
> +	  Legacy user code that does not know how to find the vDSO expects
> +	  to be able to issue three syscalls by calling fixed addresses in
> +	  kernel space. Since this location is not randomized with ASLR,
> +	  it can be used to assist security vulnerability exploitation.
> +
> +	  This setting can be changed at boot time via the kernel command
> +	  line parameter vsyscall=[native|emulate|none].
> +
> +	  On a system with recent enough glibc (2.14 or newer) and no
> +	  static binaries, you can say None without a performance penalty
> +	  to improve security.
> +
> +	  If unsure, select "Emulate".
> +
> +	config LEGACY_VSYSCALL_NATIVE
> +		bool "Native"
> +		help
> +		  Actual executable code is located in the fixed vsyscall
> +		  address mapping, implementing time() efficiently. Since
> +		  this makes the mapping executable, it can be used during
> +		  security vulnerability exploitation (traditionally as
> +		  ROP gadgets). This configuration is not recommended.
> +
> +	config LEGACY_VSYSCALL_EMULATE
> +		bool "Emulate"
> +		help
> +		  The kernel traps and emulates calls into the fixed
> +		  vsyscall address mapping. This makes the mapping
> +		  non-executable, but it still contains known contents,
> +		  which could be used in certain rare security vulnerability
> +		  exploits. This configuration is recommended when userspace
> +		  still uses the vsyscall area.
> +
> +	config LEGACY_VSYSCALL_NONE
> +		bool "None"
> +		help
> +		  There will be no vsyscall mapping at all. This will
> +		  eliminate any risk of ASLR bypass due to the vsyscall
> +		  fixed address mapping. Attempts to use the vsyscalls
> +		  will be reported to dmesg, so that either old or
> +		  malicious userspace programs can be identified.
> +
> +endchoice
> +
>  config CMDLINE_BOOL
>  	bool "Built-in kernel command line"
>  	---help---
> diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c b/arch/x86/entry/vsyscall/vsyscall_64.c
> index 2dcc6ff6fdcc..47e2904b043b 100644
> --- a/arch/x86/entry/vsyscall/vsyscall_64.c
> +++ b/arch/x86/entry/vsyscall/vsyscall_64.c
> @@ -38,7 +38,14 @@
>  #define CREATE_TRACE_POINTS
>  #include "vsyscall_trace.h"
>  
> -static enum { EMULATE, NATIVE, NONE } vsyscall_mode = EMULATE;
> +static enum { EMULATE, NATIVE, NONE } vsyscall_mode =
> +#ifdef CONFIG_LEGACY_VSYSCALL_NATIVE
> +	NATIVE;
> +#elif CONFIG_LEGACY_VSYSCALL_NONE
> +	NONE;
> +#else
> +	EMULATE;
> +#endif
>  
>  static int __init vsyscall_setup(char *str)
>  {
> -- 
> 1.9.1
> 
> 
> -- 
> Kees Cook
> Chrome OS Security