[PATCH BACKPORT v2 3.4 - 4.5] x86/traps: Ignore high word of regs->cs in early_idt_handler_common

[PATCH BACKPORT v2 3.4 - 4.5] x86/traps: Ignore high word of regs->cs in early_idt_handler_common

[Andy Lutomirski]

This is a backport of:
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>
---
 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 af1112980dd4..99373aa0762a 100644
--- a/arch/x86/kernel/head_32.S
+++ b/arch/x86/kernel/head_32.S
@@ -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
-- 
2.9.3

Re: [PATCH BACKPORT v2 3.4 - 4.5] x86/traps: Ignore high word of regs->cs in early_idt_handler_common

[Greg KH]

On Thu, Dec 01, 2016 at 09:26:42AM -0800, Andy Lutomirski wrote:
> This is a backport of:
> 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. ]

Thanks for the backport, now queued up for 4.4-stable.

greg k-h

Re: [PATCH BACKPORT v2 3.4 - 4.5] x86/traps: Ignore high word of regs->cs in early_idt_handler_common

[Ben Hutchings]

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On Thu, 2016-12-01 at 09:26 -0800, Andy Lutomirski wrote:
> This is a backport of:
> 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. ]
[...]

Queued up for 3.16, thanks.

Ben.

-- 
Ben Hutchings
Lowery's Law:
             If it jams, force it. If it breaks, it needed replacing
anyway.


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