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

[PATCH BACKPORT 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 cb32c69920e58a1a58e7b5cad975038a69c0ce7d 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 commit 1e02ce4cccdc ("x86:
Store a per-cpu shadow copy of CR4") and commit 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 3.4 - 4.5] x86/traps: Ignore high word of regs->cs in early_idt_handler_common

[Jiri Slaby]

On 11/30/2016, 09:37 PM, Andy Lutomirski wrote:
> This is a backport of:
> commit cb32c69920e58a1a58e7b5cad975038a69c0ce7d upstream

This is in fact fc0e81b2bea0ebceb71889b61d2240856141c9ee, right?

And it appears to have a different commit log :/.

Can we have that corrected?

> 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 commit 1e02ce4cccdc ("x86:
> Store a per-cpu shadow copy of CR4") and commit 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
> 

thanks,
-- 
js
suse labs

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

[Willy Tarreau]

On Wed, Nov 30, 2016 at 12:37:53PM -0800, Andy Lutomirski wrote:
> This is a backport of:
> commit cb32c69920e58a1a58e7b5cad975038a69c0ce7d 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 commit 1e02ce4cccdc ("x86:
> Store a per-cpu shadow copy of CR4") and commit 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. ]

This morning before coffee took effect I read 3.4-3.5 so I classed
it, now I've queued it for 3.10.

Thanks Andy!
Willy

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

[Andy Lutomirski]

On Thu, Dec 1, 2016 at 12:48 AM, Jiri Slaby <jslaby@suse.cz> wrote:
> On 11/30/2016, 09:37 PM, Andy Lutomirski wrote:
>> This is a backport of:
>> commit cb32c69920e58a1a58e7b5cad975038a69c0ce7d upstream
>
> This is in fact fc0e81b2bea0ebceb71889b61d2240856141c9ee, right?
>
> And it appears to have a different commit log :/.
>
> Can we have that corrected?

Sorry -- I grabbed the wrong git version.  The diff is correct but the
changelog was indeed out of date.  I'll send v2.  There will still be
small changelog differences because the buggy function is in a
different place and has a different name.

--Andy