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Subject[PATCH 10/24] x86/mm/kaiser: Map the entry stack variables
From: Dave Hansen <>

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 <>
Signed-off-by: Thomas Gleixner <>
Cc: Andy Lutomirski <>
Cc: Borislav Petkov <>
Cc: Brian Gerst <>
Cc: Denys Vlasenko <>
Cc: H. Peter Anvin <>
Cc: Josh Poimboeuf <>
Cc: Linus Torvalds <>
Cc: Peter Zijlstra <>
Cc: Rik van Riel <>
Signed-off-by: Ingo Molnar <>
arch/x86/kernel/cpu/common.c | 2 +-
arch/x86/kernel/process_64.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 7b348cc33e2d..f4f4ab8525bd 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1515,7 +1515,7 @@ 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 --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 157f81816915..631e229ab428 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -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)
 \ /
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