[3.8.y.z extended stable] Patch "x86: Ignore NMIs that come in during early boot" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    x86: Ignore NMIs that come in during early boot

to the linux-3.8.y-queue branch of the 3.8.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.8.y-queue

This patch is scheduled to be released in version 3.8.13.21.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.8.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 2955b99c1d9593d9e853eb572ab26c5779bf89a5 Mon Sep 17 00:00:00 2001
From: "H. Peter Anvin" <hpa at linux.intel.com>
Date: Fri, 7 Mar 2014 15:05:20 -0800
Subject: x86: Ignore NMIs that come in during early boot

commit 5fa10196bdb5f190f595ebd048490ee52dddea0f upstream.

Don Zickus reports:

A customer generated an external NMI using their iLO to test kdump
worked.  Unfortunately, the machine hung.  Disabling the nmi_watchdog
made things work.

I speculated the external NMI fired, caused the machine to panic (as
expected) and the perf NMI from the watchdog came in and was latched.
My guess was this somehow caused the hang.

   ----

It appears that the latched NMI stays latched until the early page
table generation on 64 bits, which causes exceptions to happen which
end in IRET, which re-enable NMI.  Therefore, ignore NMIs that come in
during early execution, until we have proper exception handling.

Reported-and-tested-by: Don Zickus <dzickus at redhat.com>
Link: http://lkml.kernel.org/r/1394221143-29713-1-git-send-email-dzickus@redhat.com
Signed-off-by: H. Peter Anvin <hpa at linux.intel.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 arch/x86/kernel/head_32.S | 7 ++++++-
 arch/x86/kernel/head_64.S | 6 +++++-
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index c8932c7..53c38f9 100644
--- a/arch/x86/kernel/head_32.S
+++ b/arch/x86/kernel/head_32.S
@@ -576,6 +576,10 @@ ENDPROC(early_idt_handlers)
 	/* This is global to keep gas from relaxing the jumps */
 ENTRY(early_idt_handler)
 	cld
+
+	cmpl $X86_TRAP_NMI,(%esp)
+	je is_nmi		# Ignore NMI
+
 	cmpl $2,%ss:early_recursion_flag
 	je hlt_loop
 	incl %ss:early_recursion_flag
@@ -626,8 +630,9 @@ ex_entry:
 	pop %edx
 	pop %ecx
 	pop %eax
-	addl $8,%esp		/* drop vector number and error code */
 	decl %ss:early_recursion_flag
+is_nmi:
+	addl $8,%esp		/* drop vector number and error code */
 	iret
 ENDPROC(early_idt_handler)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 980053c..7dd8d41 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -306,6 +306,9 @@ early_idt_handlers:
 ENTRY(early_idt_handler)
 	cld

+	cmpl $X86_TRAP_NMI,(%rsp)
+	je is_nmi		# Ignore NMI
+
 	cmpl $2,early_recursion_flag(%rip)
 	jz  1f
 	incl early_recursion_flag(%rip)
@@ -360,8 +363,9 @@ ENTRY(early_idt_handler)
 	popq %rdx
 	popq %rcx
 	popq %rax
-	addq $16,%rsp		# drop vector number and error code
 	decl early_recursion_flag(%rip)
+is_nmi:
+	addq $16,%rsp		# drop vector number and error code
 	INTERRUPT_RETURN

 	.balign 4
--
1.8.3.2