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SubjectRe: [PATCH 24/30] x86, kaiser: disable native VSYSCALL
On Wed, Nov 8, 2017 at 11:47 AM, Dave Hansen
<dave.hansen@linux.intel.com> wrote:
>
> From: Dave Hansen <dave.hansen@linux.intel.com>
>
> 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@linux.intel.com>
> Cc: Moritz Lipp <moritz.lipp@iaik.tugraz.at>
> Cc: Daniel Gruss <daniel.gruss@iaik.tugraz.at>
> Cc: Michael Schwarz <michael.schwarz@iaik.tugraz.at>
> Cc: Richard Fellner <richard.fellner@student.tugraz.at>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Kees Cook <keescook@google.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: x86@kernel.org
>
> ---
>
> 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
> _

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