linux-kernel - Re: [crash] PANIC: double fault, error

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Date:   Sat, 25 Nov 2017 10:21:13 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Andy Lutomirski <luto@...capital.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [crash] PANIC: double fault, error_code: 0x0


* Ingo Molnar <mingo@...nel.org> wrote:

> 
> * Andy Lutomirski <luto@...capital.net> wrote:
> 
> > > Note that if *any* of those 4 padding sequences is removed, the kernel starts 
> > > crashing again. Also note that the exact size of the padding appears to be not 
> > > material - it could be larger as well, i.e. it's not an alignment bug I think.
> > > 
> > > In any case it's not a problem in the actual assembly code paths itself it 
> > > appears.
> > > 
> > > One guess would be tha it's some sort of sizing bug: maybe the padding forces a 
> > > key piece of data or code on another page - but I'm too tired to root cause it 
> > > right now.
> > > 
> > > Any ideas?
> > 
> > This smells like a pagerable setup bug. Either the pagetables are a bit broken or they're totally busted and the passing gets something in a more TLB-friendly place.
> 
> Also note that the delta patch below also keeps it working, i.e. doubling the 
> first padding and eliminating the second padding.
> 
> I.e. it's the total per IRQ entry padding that matters, not the exact placement of 
> the padding.
> 
> I.e. some sort of sizing bug - IDT and/or the pagetables.
> 
> (Also note that in my config NR_CPUS is at 128 - defconfigs are 64.)

The simplest padding I found is the one below - this indicates some sort of 
section sizing or page table setup bug (or page alignment bug) and makes races and 
other bugs less likely.

Thanks,

	Ingo

=================>
 arch/x86/entry/entry_64.S | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 4ac952080869..ea992ca4e74f 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -547,6 +547,8 @@ END(irq_entries_start)
 	ud2
 .Lokay_\@:
 	addq $8, %rsp
+#else
+	.rep 64; nop; .endr
 #endif
 .endm