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Subject[PATCH 3.12 20/38] x86/traps: Ignore high word of regs->cs in early_fixup_exception()
From: Andy Lutomirski <>

3.12-stable review patch. If anyone has any objections, please let me know.


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. ]

Fixes: 4c5023a3fa2e ("x86-32: Handle exception table entries during early boot")
Cc: H. Peter Anvin <>
Reported-by: Matthew Whitehead <>
Signed-off-by: Andy Lutomirski <>
Signed-off-by: Jiri Slaby <>
arch/x86/kernel/head_32.S | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index 30a2aa3782fa..879e67acf463 100644
--- a/arch/x86/kernel/head_32.S
+++ b/arch/x86/kernel/head_32.S
@@ -564,7 +564,7 @@ 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
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