x86/traps: Ignore high word of regs->cs in early_fixup_exception()
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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index 6bc9ae2..8f1a3f4 100644
--- a/arch/x86/kernel/head_32.S
+++ b/arch/x86/kernel/head_32.S
@@ -571,7 +571,7 @@
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
