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>
Cc: stable@vger.kernel.org
Reported-by: Matthew Whitehead <tedheadster@gmail.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index 30a2aa3782fafd53998d131be23c7564482c5d1c..879e67acf463729f7d5509993651c23d1543390f 100644 (file)
--- 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