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x86/traps: Ignore high word of regs->cs in early_fixup_exception()

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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:

  1e02ce4ccc ("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: 4c5023a3fa ("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>
This commit is contained in:
Andy Lutomirski 2016-11-19 18:42:40 -08:00 • committed by Greg Kroah-Hartman
parent 995761627d
commit 5a5f703019
1 changed files with 1 additions and 1 deletions
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2
arch/x86/kernel/head_32.S
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@ -571,7 +571,7 @@ 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