There are times that we enter the kernel and do not have a safe
stack, like at SYSCALL entry. We use the per-cpu vairables
'rsp_scratch' and 'cpu_current_top_of_stack' to save off the old
%rsp and find a safe place to have a stack.
You can not directly manipulate the CR3 register. You can only
'MOV' to it from another register, which means we need to clobber
a register in order to do any CR3 manipulation. User-mapping these
variables allows us to obtain a safe stack *before* we switch the
CR3 value.
Signed-off-by: Dave Hansen <dave.han...@linux.intel.com>
Cc: Moritz Lipp <moritz.l...@iaik.tugraz.at>
Cc: Daniel Gruss <daniel.gr...@iaik.tugraz.at>
Cc: Michael Schwarz <michael.schw...@iaik.tugraz.at>
Cc: Andy Lutomirski <l...@kernel.org>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: Kees Cook <keesc...@google.com>
Cc: Hugh Dickins <hu...@google.com>
Cc: x...@kernel.org
---
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
2017-10-31 15:03:56.168388378 -0700
+++ b/arch/x86/kernel/cpu/common.c 2017-10-31 15:03:56.175388709 -0700
@@ -1440,7 +1440,7 @@ 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;
EXPORT_PER_CPU_SYMBOL(cpu_current_top_of_stack);
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
2017-10-31 15:03:56.170388472 -0700
+++ b/arch/x86/kernel/process_64.c 2017-10-31 15:03:56.176388756 -0700
@@ -59,7 +59,7 @@
#include <asm/unistd_32_ia32.h>
#endif
-__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)
_
