linux-kernel - Re: [PATCH v2 6/8] x86/entry: add unwind hint annotations

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Date:   Thu, 29 Jun 2017 14:05:59 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        live-patching@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jiri Slaby <jslaby@...e.cz>, Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mike Galbraith <efault@....de>
Subject: Re: [PATCH v2 6/8] x86/entry: add unwind hint annotations

On Thu, Jun 29, 2017 at 11:50:18AM -0700, Andy Lutomirski wrote:
> On Thu, Jun 29, 2017 at 10:53 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > There's a bug here that will need a small change to the entry code.
> >
> > Mike Galbraith reported:
> >
> >   WARNING: can't dereference registers at ffffc900089d7e08 for ip ffffffff81740bbb
> >
> > After some looking I found that it's caused by the following code
> > snippet in the 'interrupt' macro in entry_64.S:
> >
> >         /*
> >          * Save previous stack pointer, optionally switch to interrupt stack.
> >          * irq_count is used to check if a CPU is already on an interrupt stack
> >          * or not. While this is essentially redundant with preempt_count it is
> >          * a little cheaper to use a separate counter in the PDA (short of
> >          * moving irq_enter into assembly, which would be too much work)
> >          */
> >         movq    %rsp, %rdi
> >         incl    PER_CPU_VAR(irq_count)
> >         cmovzq  PER_CPU_VAR(irq_stack_ptr), %rsp
> >         UNWIND_HINT_REGS base=rdi
> >         pushq   %rdi
> >         UNWIND_HINT_REGS indirect=1
> >
> > The problem is that it's changing the stack pointer *before* writing the
> > previous stack pointer (push %rdi).  So when unwinding from an NMI which
> > hit between the rsp write and the rdi push, the unwinder tries to access
> > the regs on the previous stack (by reading rdi), but the previous stack
> > pointer isn't there yet, so the access is considered out of bounds.
> 
> Ugh, that code.  Does this problem go away with this patch applied:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/entry_ist&id=2231ec7e0bcc1a2bc94a17081511ab54cc6badd1
> 
> If so, want to update the patch for new kernels (shouldn't conflict
> with anything except your unwind hints)?

I don't think that patch will fix it, because it still updates rsp
*before* writing the old rsp on the new stack.  So there's still a
window where the "previous stack" pointer is missing.

-- 
Josh