[PATCH 11/23] x86, kaiser: map entry stack variables

From: Dave Hansen
Date: Wed Nov 22 2017 - 19:36:12 EST

From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

There are times where the kernel is entered but there is not a
safe stack, like at SYSCALL entry. To obtain a safe stack, the
per-cpu variables 'rsp_scratch' and 'cpu_current_top_of_stack'
are used to save the old %rsp value and to find where the kernel
stack should start.

You can not directly manipulate the CR3 register. You can only
'MOV' to it from another register, which means a register must be
clobbered in order to do any CR3 manipulation. User-mapping
these variables allows us to obtain a safe stack and use it for
temporary storage *before* CR3 is switched.

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/kernel/cpu/common.c | 2 +-
b/arch/x86/kernel/process_64.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)

diff -puN arch/x86/kernel/cpu/common.c~kaiser-user-map-stack-helper-vars arch/x86/kernel/cpu/common.c
--- a/arch/x86/kernel/cpu/common.c~kaiser-user-map-stack-helper-vars 2017-11-22 15:45:50.128619736 -0800
+++ b/arch/x86/kernel/cpu/common.c 2017-11-22 15:45:50.134619736 -0800
@@ -1524,7 +1524,7 @@ EXPORT_PER_CPU_SYMBOL(__preempt_count);
* the top of the kernel stack. Use an extra percpu variable to track the
* top of the kernel stack directly.
-DEFINE_PER_CPU(unsigned long, cpu_current_top_of_stack) =
+DEFINE_PER_CPU_USER_MAPPED(unsigned long, cpu_current_top_of_stack) =
(unsigned long)&init_thread_union + THREAD_SIZE;

diff -puN arch/x86/kernel/process_64.c~kaiser-user-map-stack-helper-vars arch/x86/kernel/process_64.c
--- a/arch/x86/kernel/process_64.c~kaiser-user-map-stack-helper-vars 2017-11-22 15:45:50.130619736 -0800
+++ b/arch/x86/kernel/process_64.c 2017-11-22 15:45:50.134619736 -0800
@@ -59,7 +59,7 @@
#include <asm/unistd_32_ia32.h>

-__visible DEFINE_PER_CPU(unsigned long, rsp_scratch);
+__visible DEFINE_PER_CPU_USER_MAPPED(unsigned long, rsp_scratch);

/* Prints also some state that isn't saved in the pt_regs */
void __show_regs(struct pt_regs *regs, int all)