Re: [patch 01/14] x86/irq/64: Limit IST stack overflow check to #DB stack

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Sun, Mar 31, 2019 at 11:40:21PM +0200, Thomas Gleixner wrote:
> 37fe6a42b343 ("x86: Check stack overflow in detail") added a broad check
> for the full exception stack area, i.e. it considers the full exception
> stack are as valid.
        ^^^
        area

> 
> That's wrong in two aspects:
> 
>  1) It does not check the individual areas one by one
> 
>  2) #DF, NMI and #MCE are not enabling interrupts which means that a
>     regular device interrupt cannot happen in their context. In fact if a
>     device interrupt hits one of those IST stacks that's a bug because some
>     code path enabled interrupts while handling the exception.
> 
> Limit the check to the #DB stack and consider all other IST stacks as
> 'overflow' or invalid.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Mitsuo Hayasaka <mitsuo.hayasaka.hu@hitachi.com>
> ---
>  arch/x86/kernel/irq_64.c |   19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
> 
> --- a/arch/x86/kernel/irq_64.c
> +++ b/arch/x86/kernel/irq_64.c
> @@ -26,9 +26,18 @@ int sysctl_panic_on_stackoverflow;
>  /*
>   * Probabilistic stack overflow check:
>   *
> - * Only check the stack in process context, because everything else
> - * runs on the big interrupt stacks. Checking reliably is too expensive,
> - * so we just check from interrupts.
> + * Regular device interrupts can enter on the following stacks:
> + *
> + * - User stack
> + *
> + * - Kernel task stack
> + *
> + * - Interrupt stack if a device driver reenables interrupt
> + *   which should only happen in really old drivers.
> + *
> + * - Debug IST stack
> + *
> + * All other contexts are invalid.
>   */
>  static inline void stack_overflow_check(struct pt_regs *regs)
>  {
> @@ -53,8 +62,8 @@ static inline void stack_overflow_check(
>  		return;
>  
>  	oist = this_cpu_ptr(&orig_ist);
> -	estack_top = (u64)oist->ist[0] - EXCEPTION_STKSZ + STACK_TOP_MARGIN;
> -	estack_bottom = (u64)oist->ist[N_EXCEPTION_STACKS - 1];
> +	estack_bottom = (u64)oist->ist[DEBUG_STACK];
> +	estack_top = estack_bottom - DEBUG_STKSZ + STACK_TOP_MARGIN;
>  	if (regs->sp >= estack_top && regs->sp <= estack_bottom)
>  		return;
>  
> 
>