tree 02daa076bd0daaa116b0dc0d1821d88d0dc70262
parent 995761627d97cc374f14203589b3a6385019bf0b
author Andy Lutomirski <luto@kernel.org> 1479609760 -0800
committer Greg Kroah-Hartman <gregkh@linuxfoundation.org> 1481177724 +0100

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>