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Date:   Fri, 24 Nov 2017 23:09:34 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        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:

> 
> * Ingo Molnar <mingo@...nel.org> wrote:
> 
> > This is a repost of the latest entry-stack plus Kaiser bits from Andy Lutomirski
> > (v3 series from today) and Dave Hansen (kaiser-414-tipwip-20171123 version),
> > on top of latest tip:x86/urgent (12a78d43de76).
> > 
> > This version is pretty well tested, at least on the usual x86 tree test systems.
> > It has a couple of merge mistakes fixed, the biggest difference is in patch #22:
> > 
> >   x86/mm/kaiser: Prepare assembly for entry/exit CR3 switching
> > 
> > The other patches are identical or very close to what I posted earlier today.
> 
> Here's a new bug, on a testsystem I get the double fault boot crash attached 
> below. The same bzImage crashes on other systems as well, so it's not CPU 
> dependent.
> 
> Via Kconfig-bisection I have narrowed it down to the following .config detail: 
> it's triggered by _disabling_ CONFIG_DEBUG_ENTRY and enabling CONFIG_KAISER=y.
> 
> I.e. one of the sanity checks of CONFIG_DEBUG_ENTRY has some positive side effect. 
> I'll try to track down which one it is - any ideas meanwhile?
> 
> Thanks,
> 
> 	Ingo
> 
> [    8.797733] calling  pt_dump_init+0x0/0x3b @ 1
> [    8.803144] initcall pt_dump_init+0x0/0x3b returned 0 after 1 usecs
> [    8.810589] calling  aes_init+0x0/0x11 @ 1
> [    8.815757] initcall aes_init+0x0/0x11 returned 0 after 141 usecs
> [    8.823020] calling  ghash_pclmulqdqni_mod_init+0x0/0x54 @ 1
> [    8.831002] PANIC: double fault, error_code: 0x0
> [    8.831002] CPU: 11 PID: 260 Comm: modprobe Not tainted 4.14.0-01419-g1b46550a680d-dirty #17
> [    8.831002] Hardware name: Intel Corporation S2600GZ/S2600GZ, BIOS SE5C600.86B.02.02.0002.122320131210 12/23/2013
> [    8.831002] task: ffff880828ba8000 task.stack: ffffc90004444000
> [    8.831002] RIP: 0010:page_fault+0x11/0x60
> [    8.831002] RSP: 0000:ffffffffff0e7fc8 EFLAGS: 00010046
> [    8.831002] RAX: 00000000819d4d77 RBX: 0000000000000001 RCX: ffffffff819d4d77

After much more debugging, the patch below 'fixes' the crash as well, when 
CONFIG_DEBUG_ENTRY is disabled.

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?

Thanks,

	Ingo
---
 arch/x86/entry/entry_64.S | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 4ac952080869..e83029892017 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 16; nop; .endr
 #endif
 .endm
 
@@ -597,6 +599,8 @@ END(irq_entries_start)
 	je	.Lirq_stack_okay\@
 	ud2
 	.Lirq_stack_okay\@:
+#else
+	.rep 16; nop; .endr
 #endif
 
 .Lirq_stack_push_old_rsp_\@:
@@ -707,6 +711,8 @@ GLOBAL(swapgs_restore_regs_and_return_to_usermode)
 	jnz	1f
 	ud2
 1:
+#else
+	.rep 16; nop; .endr
 #endif
 	POP_EXTRA_REGS
 	popq	%r11
@@ -773,6 +779,8 @@ GLOBAL(restore_regs_and_return_to_kernel)
 	jz	1f
 	ud2
 1:
+#else
+	.rep 16; nop; .endr
 #endif
 	POP_EXTRA_REGS
 	POP_C_REGS

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