x86: Ignore NMIs that come in during early boot

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@redhat.com>
Link: http://lkml.kernel.org/r/1394221143-29713-1-git-send-email-dzickus@redhat.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: <stable@vger.kernel.org> # v3.5+, older with some backport effort

2 files changed, 11 insertions, 2 deletions

diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index 81ba27679f18..d2a21590794a 100644
--- a/arch/x86/kernel/head_32.S
+++ b/arch/x86/kernel/head_32.S
@@ -544,6 +544,10 @@ 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
@@ -594,8 +598,9 @@ 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 e1aabdb314c8..33f36c78594e 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -343,6 +343,9 @@ 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)
@@ -405,8 +408,9 @@ 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
 ENDPROC(early_idt_handler)