Re: [PATCH v8 5/6] x86/signal: Detect and prevent an alternate signal stack overflow

[Bae, Chang Seok]

On Apr 22, 2021, at 15:04, David Laight <David.Laight@xxxxxxxxxx> wrote:
> From: Bae, Chang Seok Sent: 22 April 2021 17:31
>> 
>> On Apr 22, 2021, at 01:46, David Laight <David.Laight@xxxxxxxxxx> wrote:
>>> From: Chang S. Bae Sent: 22 April 2021 05:49
>>>> 
>>>> 
>>>> diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
>>>> index 3f6a0fcaa10c..ae60f838ebb9 100644
>>>> --- a/include/linux/sched/signal.h
>>>> +++ b/include/linux/sched/signal.h
>>>> @@ -537,6 +537,17 @@ static inline int kill_cad_pid(int sig, int priv)
>>>> #define SEND_SIG_NOINFO ((struct kernel_siginfo *) 0)
>>>> #define SEND_SIG_PRIV	((struct kernel_siginfo *) 1)
>>>> 
>>>> +static inline int __on_sig_stack(unsigned long sp)
>>>> +{
>>>> +#ifdef CONFIG_STACK_GROWSUP
>>>> +	return sp >= current->sas_ss_sp &&
>>>> +		sp - current->sas_ss_sp < current->sas_ss_size;
>>>> +#else
>>>> +	return sp > current->sas_ss_sp &&
>>>> +		sp - current->sas_ss_sp <= current->sas_ss_size;
>>>> +#endif
>>>> +}
>>>> +
>>> 
>>> Those don't look different enough.
>> 
>> The difference is on the SS_AUTODISARM flag check.  This refactoring was
>> suggested as on_sig_stack() brought confusion [3].
> 
> I was just confused by the #ifdef.
> Whether %sp points to the last item or the next space is actually
> independent of the stack direction.
> A stack might usually use pre-decrement and post-increment but it
> doesn't have to.
> The stack pointer can't be right at one end of the alt-stack
> area (because that is the address you'd use when you switch to it),
> and if you are any where near the other end you are hosed.
> So a common test:
> 	return (unsigned long)(sp - current->sas_ss_sp) < current->sas_ss_size;
> will always work.
> 
> It isn't as though the stack pointer should be anywhere else
> other than the 'real' thread stack.

Thanks for the suggestion. Yes, this hunk can be made better like that. But I
would make this change as pure refactoring. Perhaps, follow up after this
series.

Chang