linux-kernel - Re: [PATCH 13/43] x86/asm/64: Use a percpu trampoline stack for IDT entries

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Date:   Sun, 26 Nov 2017 15:16:47 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...capital.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 13/43] x86/asm/64: Use a percpu trampoline stack for IDT
 entries


* Borislav Petkov <bp@...en8.de> wrote:

> On Fri, Nov 24, 2017 at 06:23:41PM +0100, Ingo Molnar wrote:
> > From: Andy Lutomirski <luto@...nel.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@...nel.org>
> > Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> 
> I think you mean Reviewed-by: here. The following patches have it too.
> 
> > Cc: Borislav Petkov <bpetkov@...e.de>
> > Cc: Brian Gerst <brgerst@...il.com>
> > Cc: Dave Hansen <dave.hansen@...el.com>
> > Cc: Josh Poimboeuf <jpoimboe@...hat.com>
> > Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Link: https://lkml.kernel.org/r/fa3958723a1a85baeaf309c735b775841205800e.1511497875.git.luto@kernel.org
> > Signed-off-by: Ingo Molnar <mingo@...nel.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.

Ok, I made it:

        /* On x86_64, sp0 always points to the entry trampoline stack, which is constant: */
#ifdef CONFIG_X86_32
        load_sp0(task->thread.sp0);
#endif

> >  	/*
> >  	 * 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@...e.de>

Fixed those details and added your tag, thanks Boris!

	Ingo