linux-kernel - Re: [PATCH 12/43] x86/espfix/64: Stop assuming that pt

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Date:   Sun, 26 Nov 2017 15:05:33 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Borislav Petkov <bp@...en8.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        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 12/43] x86/espfix/64: Stop assuming that pt_regs is on
 the entry stack


* Andy Lutomirski <luto@...nel.org> wrote:

> On Fri, Nov 24, 2017 at 10:25 AM, Borislav Petkov <bp@...en8.de> wrote:
> > On Fri, Nov 24, 2017 at 06:23:40PM +0100, Ingo Molnar wrote:
> >> From: Andy Lutomirski <luto@...nel.org>
> >>
> >> When we start using an entry trampoline, a #GP from userspace will
> >> be delivered on the entry stack, not on the task stack.  Fix the
> >> espfix64 #DF fixup to set up #GP according to TSS.SP0, rather than
> >> assuming that pt_regs + 1 == SP0.  This won't change anything
> >> without an entry stack, but it will make the code continue to work
> >> when an entry stack is added.
> >>
> >> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> >> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> >> Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
> >> 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/b1ef4136616c6bd2a75d1fd2736d1d54437d65a8.1511497875.git.luto@kernel.org
> >> Signed-off-by: Ingo Molnar <mingo@...nel.org>
> >> ---
> >>  arch/x86/kernel/traps.c | 5 +++--
> >>  1 file changed, 3 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> >> index 2008dd0f8ccb..1bd43f044c62 100644
> >> --- a/arch/x86/kernel/traps.c
> >> +++ b/arch/x86/kernel/traps.c
> >> @@ -359,7 +359,8 @@ dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
> >>               regs->cs == __KERNEL_CS &&
> >>               regs->ip == (unsigned long)native_irq_return_iret)
> >>       {
> >> -             struct pt_regs *normal_regs = task_pt_regs(current);
> >> +             struct pt_regs *normal_regs =
> >> +                     (struct pt_regs *)this_cpu_read(cpu_tss.x86_tss.sp0) - 1;
> >
> > Just let that line stick out. Also, you can shorten it by renaming
> > normal_regs to something much shorter - it is a local variable and the
> > comment already explains everything you you can just as well have:
> >
> >          struct pt_regs *r = (struct pt_regs *)this_cpu_read(cpu_tss.x86_tss.sp0) - 1;
> >
> 
> Done, along with much better comments.
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/entry_stack&id=6510485d026abdd144d170b1bc8508327acb5648

I have added Boris's reviewed-by tag:

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

which I suspect applies now?

Thanks,

	Ingo