Re: [PATCH 24/30] x86, kaiser: disable native VSYSCALL

[Andy Lutomirski]

On Wed, Nov 8, 2017 at 11:47 AM, Dave Hansen
<dave.hansen@xxxxxxxxxxxxxxx> wrote:
>
> From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>
> The VSYSCALL page is mapped by kernel page tables at a kernel address.
> It is troublesome to support with KAISER in place, so disable the
> native case.
>
> Also add some help text about how KAISER might affect the emulation
> case as well.

Can you re-explain why this is helpful?

Also, I'm about to send patches that may cause a rethinking of how
KAISER handles the fixmap.

--Andy

>
> Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Moritz Lipp <moritz.lipp@xxxxxxxxxxxxxx>
> Cc: Daniel Gruss <daniel.gruss@xxxxxxxxxxxxxx>
> Cc: Michael Schwarz <michael.schwarz@xxxxxxxxxxxxxx>
> Cc: Richard Fellner <richard.fellner@xxxxxxxxxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: x86@xxxxxxxxxx
>
> ---
>
>  b/arch/x86/Kconfig |    8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff -puN arch/x86/Kconfig~kaiser-no-vsyscall arch/x86/Kconfig
> --- a/arch/x86/Kconfig~kaiser-no-vsyscall       2017-11-08 10:45:39.157681370 -0800
> +++ b/arch/x86/Kconfig  2017-11-08 10:45:39.162681370 -0800
> @@ -2231,6 +2231,9 @@ choice
>
>         config LEGACY_VSYSCALL_NATIVE
>                 bool "Native"
> +               # The VSYSCALL page comes from the kernel page tables
> +               # and is not available when KAISER is enabled.
> +               depends on ! KAISER
>                 help
>                   Actual executable code is located in the fixed vsyscall
>                   address mapping, implementing time() efficiently. Since
> @@ -2248,6 +2251,11 @@ choice
>                   exploits. This configuration is recommended when userspace
>                   still uses the vsyscall area.
>
> +                 When KAISER is enabled, the vsyscall area will become
> +                 unreadable.  This emulation option still works, but KAISER
> +                 will make it harder to do things like trace code using the
> +                 emulation.
> +
>         config LEGACY_VSYSCALL_NONE
>                 bool "None"
>                 help
> _