On Wed, Nov 22, 2017 at 4:34 PM, Dave Hansen <[email protected]> wrote: > > From: Dave Hansen <[email protected]> > > 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 <[email protected]> > Cc: Moritz Lipp <[email protected]> > Cc: Daniel Gruss <[email protected]> > Cc: Michael Schwarz <[email protected]> > Cc: Richard Fellner <[email protected]> > Cc: Andy Lutomirski <[email protected]> > Cc: Linus Torvalds <[email protected]> > Cc: Kees Cook <[email protected]> > Cc: Hugh Dickins <[email protected]> > Cc: [email protected] > --- > > 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;
This is in an x86_32-only section and should be dropped, I think. > 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> > #endif > > -__visible DEFINE_PER_CPU(unsigned long, rsp_scratch); > +__visible DEFINE_PER_CPU_USER_MAPPED(unsigned long, rsp_scratch); > This shouldn't be needed any more either.
