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

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 <hpa@zytor.com>
Reported-by: Matthew Whitehead <tedheadster@gmail.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
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
--
2.11.0
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