Re: [PATCH 5/5] x86/vsyscall: Change the default vsyscall mode to xonly

[Kees Cook]

On Mon, Jun 10, 2019 at 01:25:31PM -0700, Andy Lutomirski wrote:
> The use case for full emulation over xonly is very esoteric.  Let's
> change the default to the safer xonly mode.

Perhaps describe the esoteric cases here (and maybe in the Kconfig help
text)? That should a user determine if they actually need it. (What
would the failure under xonly look like for someone needing emulate?)

-Kees

> 
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Kernel Hardening <kernel-hardening@xxxxxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> ---
>  arch/x86/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 054033cc4b1b..e56f33e6b045 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -2280,7 +2280,7 @@ config COMPAT_VDSO
>  choice
>  	prompt "vsyscall table for legacy applications"
>  	depends on X86_64
> -	default LEGACY_VSYSCALL_EMULATE
> +	default LEGACY_VSYSCALL_XONLY
>  	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
> -- 
> 2.21.0
> 

-- 
Kees Cook