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

[Borislav Petkov]

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.
> 
> 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@xxxxxxxxxxxxx>
> Cc: Mitsuo Hayasaka <mitsuo.hayasaka.hu@xxxxxxxxxxx>
> ---
>  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

						     "interrupts"

> + *   which should only happen in really old drivers.
> + *
> + * - Debug IST stack
> + *
> + * All other contexts are invalid.

<---

I could use the above blurb here too, explaining why we're checking the
#DB stack only.

Otherwise:

Reviewed-by: Borislav Petkov <bp@xxxxxxx>

-- 
Regards/Gruss,
    Boris.

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