Patch "x86/traps: Ignore high word of regs->cs in early_fixup_exception()" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

    x86/traps: Ignore high word of regs->cs in early_fixup_exception()

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
x86-traps-ignore-high-word-of-regs-cs-in-early_fixup_exception.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Andy Lutomirski <[email protected]>
Date: Sat, 19 Nov 2016 18:42:40 -0800
Subject: x86/traps: Ignore high word of regs->cs in early_fixup_exception()

From: Andy Lutomirski <[email protected]>

commit fc0e81b2bea0ebceb71889b61d2240856141c9ee upstream.

On the 80486 DX, it seems that some exceptions may leave garbage in the high bits of CS. This causes sporadic failures in which early_fixup_exception() refuses to fix up an exception.

As far as I can tell, this has been buggy for a long time, but the problem seems to have been exacerbated by commits:

1e02ce4cccdc ("x86: Store a per-cpu shadow copy of CR4") e1bfc11c5a6f ("x86/init: Fix cr4_init_shadow() on CR4-less machines")

This appears to have broken for as long as we've had early exception handling.

[ This backport should apply to kernels from 3.4 - 4.5. ]

---
 arch/x86/kernel/head_32.S |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

@@ -571,7 +571,7 @@ early_idt_handler_common:
 	movl %eax,%ds
 	movl %eax,%es
 
-	cmpl $(__KERNEL_CS),32(%esp)
+	cmpw $(__KERNEL_CS),32(%esp)
 	jne 10f
 
 	leal 28(%esp),%eax	# Pointer to %eip

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/x86-traps-ignore-high-word-of-regs-cs-in-early_fixup_exception.patch