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SubjectRe: [PATCH 13/43] x86/asm/64: Use a percpu trampoline stack for IDT entries
On Fri, Nov 24, 2017 at 06:23:41PM +0100, Ingo Molnar wrote:
> From: Andy Lutomirski <luto@kernel.org>
>
> Historically, IDT entries from usermode have always gone directly
> to the running task's kernel stack. Rearrange it so that we enter on
> a percpu trampoline stack and then manually switch to the task's stack.
> This touches a couple of extra cachelines, but it gives us a chance
> to run some code before we touch the kernel stack.
>
> The asm isn't exactly beautiful, but I think that fully refactoring
> it can wait.
>
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

I think you mean Reviewed-by: here. The following patches have it too.

> Cc: Borislav Petkov <bpetkov@suse.de>
> Cc: Brian Gerst <brgerst@gmail.com>
> Cc: Dave Hansen <dave.hansen@intel.com>
> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Link: https://lkml.kernel.org/r/fa3958723a1a85baeaf309c735b775841205800e.1511497875.git.luto@kernel.org
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> ---
> arch/x86/entry/entry_64.S | 67 ++++++++++++++++++++++++++++++----------
> arch/x86/entry/entry_64_compat.S | 5 ++-
> arch/x86/include/asm/switch_to.h | 2 +-
> arch/x86/include/asm/traps.h | 1 -
> arch/x86/kernel/cpu/common.c | 6 ++--
> arch/x86/kernel/traps.c | 18 +++++------
> 6 files changed, 68 insertions(+), 31 deletions(-)

...

> diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
> index 8c6bd6863db9..a6796ac8d311 100644
> --- a/arch/x86/include/asm/switch_to.h
> +++ b/arch/x86/include/asm/switch_to.h
> @@ -93,7 +93,7 @@ static inline void update_sp0(struct task_struct *task)
> #ifdef CONFIG_X86_32
> load_sp0(task->thread.sp0);
> #else
> - load_sp0(task_top_of_stack(task));
> + /* On x86_64, sp0 always points to the entry trampoline stack. */
> #endif

You can put this comment to the one ontop and remove the #else.
ifdeffery is always ugly and the less, the better.

...

> -asmlinkage __visible notrace struct pt_regs *sync_regs(struct pt_regs *eregs)
> +asmlinkage __visible notrace
> +struct pt_regs *sync_regs(struct pt_regs *eregs)
> {
> - struct pt_regs *regs = task_pt_regs(current);
> + struct pt_regs *regs = (struct pt_regs *)this_cpu_read(cpu_current_top_of_stack) - 1;
> *regs = *eregs;
> return regs;
> }
> @@ -630,13 +630,13 @@ struct bad_iret_stack *fixup_bad_iret(struct bad_iret_stack *s)
> /*
> * This is called from entry_64.S early in handling a fault
> * caused by a bad iret to user mode. To handle the fault
> - * correctly, we want move our stack frame to task_pt_regs
> - * and we want to pretend that the exception came from the
> - * iret target.
> + * correctly, we want move our stack frame to where it would

" ... we want to move... "

> + * be had we entered directly on the entry stack (rather than
> + * just below the IRET frame) and we want to pretend that the
> + * exception came from the iret target.

s/iret/IRET/

> */
> struct bad_iret_stack *new_stack =
> - container_of(task_pt_regs(current),
> - struct bad_iret_stack, regs);
> + (struct bad_iret_stack *)this_cpu_read(cpu_tss.x86_tss.sp0) - 1;
>
> /* Copy the IRET target to the new stack. */
> memmove(&new_stack->regs.ip, (void *)s->regs.sp, 5*8);
> --

with that:

Reviewed-by: Borislav Petkov <bp@suse.de>

--
Regards/Gruss,
Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.

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