On Thu, Nov 23, 2017 at 7:37 AM, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> wrote: > On 11/22/2017 07:31 PM, Andy Lutomirski wrote: >> On Wed, Nov 22, 2017 at 4:34 PM, Dave Hansen >> <dave.hansen@xxxxxxxxxxxxxxx> wrote: >>> >>> 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; >> >> This is in an x86_32-only section and should be dropped, I think. > > It's used in entry_SYSCALL_64 (see below). But I do think it's safe to > drop now. We switch before we use it. > >>> 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. > > What about this hunk? It touches rsp_scratch before switching: > > @@ -207,9 +210,16 @@ ENTRY(entry_SYSCALL_64) > > swapgs > movq %rsp, PER_CPU_VAR(rsp_scratch) > - movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp > > - TRACE_IRQS_OFF > + /* > + * The kernel CR3 is needed to map the process stack, but we > + * need a scratch register to be able to load CR3. %rsp is > + * clobberable right now, so use it as a scratch register. > + * %rsp will be look crazy here for a couple instructions. > + */ > + SWITCH_TO_KERNEL_CR3 scratch_reg=%rsp > + > + movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp > > I'm surprised that boots, since that hunk won't execute at all. I think you should move that code into the trampoline. (Check my latest tree -- I think it's a bit off in Ingo's tree.) I've effectively split SYSCALL64 into two separate paths: entry_SYSCALL_64 (with stack switching off) and entry_SYSCALL_64_trampoline (with stack switching on). The entire point of the trampoline was to get a way to access some data that varies per cpu without needing access to traditional %gs-based percpu data. --Andy -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
