Re: [PATCH] x86/idt: Fix the X86_TRAP_BP gate

[Andrei Vagin]

On Fri, Sep 01, 2017 at 10:26:30AM +0200, Ingo Molnar wrote:
> 
> Hi Andrei,
> 
> * Andrei Vagin <avagin@xxxxxxxxxxxxx> wrote:
> 
> > Hi Thomas,
> > 
> > We run CRIU tests for linux-next and today they detected an issue. I've
> > bisected this problem and it looks like a problem is in this patch.
> 
> Ok, there appears to be a bug in that conversion - does the patch below fix the 
> regression for you?

Yes, it is.

Thank you!

> 
> Thanks,
> 
> 	Ingo
> 
> ================================>
> Subject: x86/idt: Fix the X86_TRAP_BP gate
> From: Ingo Molnar <mingo@xxxxxxxxxx>
> 
> Andrei Vagin reported a CRIU regression and bisected it back to:
> 
>   90f6225fba0c ("x86/idt: Move IST stack based traps to table init")
> 
> This table init conversion loses the system-gate property of X86_TRAP_BP
> and erroneously moves it from DPL3 to DPL0.
> 
> Fix it.
> 
> Reported-by: Andrei Vagin <avagin@xxxxxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> ---
> arch/x86/kernel/idt.c | 6 +++++-
>  arch/x86/kernel/idt.c |    6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> Index: tip/arch/x86/kernel/idt.c
> ===================================================================
> --- tip.orig/arch/x86/kernel/idt.c
> +++ tip/arch/x86/kernel/idt.c
> @@ -44,6 +44,10 @@ struct idt_data {
>  #define ISTG(_vector, _addr, _ist)			\
>  	G(_vector, _addr, _ist, GATE_INTERRUPT, DPL0, __KERNEL_CS)
>  
> +/* System interrupt gate with interrupt stack */
> +#define SISTG(_vector, _addr, _ist)			\
> +	G(_vector, _addr, _ist, GATE_INTERRUPT, DPL3, __KERNEL_CS)
> +
>  /* Task gate */
>  #define TSKG(_vector, _gdt)				\
>  	G(_vector, NULL, DEFAULT_STACK, GATE_TASK, DPL0, _gdt << 3)
> @@ -181,7 +185,7 @@ gate_desc debug_idt_table[IDT_ENTRIES] _
>  static const __initdata struct idt_data ist_idts[] = {
>  	ISTG(X86_TRAP_DB,	debug,		DEBUG_STACK),
>  	ISTG(X86_TRAP_NMI,	nmi,		NMI_STACK),
> -	ISTG(X86_TRAP_BP,	int3,		DEBUG_STACK),
> +	SISTG(X86_TRAP_BP,	int3,		DEBUG_STACK),
>  	ISTG(X86_TRAP_DF,	double_fault,	DOUBLEFAULT_STACK),
>  #ifdef CONFIG_X86_MCE
>  	ISTG(X86_TRAP_MC,	&machine_check,	MCE_STACK),