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From:   David Laight <David.Laight@ACULAB.COM>
To:     "'Chang S. Bae'" <chang.seok.bae@intel.com>,
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Subject: RE: [PATCH v8 5/6] x86/signal: Detect and prevent an alternate signal
 stack overflow
Thread-Topic: [PATCH v8 5/6] x86/signal: Detect and prevent an alternate
 signal stack overflow
Thread-Index: AQHXNzOfB++Ln2WD/U+jOvjJUzWT2qrAORRg
Date:   Thu, 22 Apr 2021 08:46:14 +0000
Message-ID: <854d6aefdf604b559e37e82669b5e67f@AcuMS.aculab.com>
References: <20210422044856.27250-1-chang.seok.bae@intel.com>
 <20210422044856.27250-6-chang.seok.bae@intel.com>
In-Reply-To: <20210422044856.27250-6-chang.seok.bae@intel.com>
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From: Chang S. Bae
> Sent: 22 April 2021 05:49
> 
> The kernel pushes context on to the userspace stack to prepare for the
> user's signal handler. When the user has supplied an alternate signal
> stack, via sigaltstack(2), it is easy for the kernel to verify that the
> stack size is sufficient for the current hardware context.
> 
> Check if writing the hardware context to the alternate stack will exceed
> it's size. If yes, then instead of corrupting user-data and proceeding with
> the original signal handler, an immediate SIGSEGV signal is delivered.

What happens if SIGSEGV is caught?

> Refactor the stack pointer check code from on_sig_stack() and use the new
> helper.
> 
> While the kernel allows new source code to discover and use a sufficient
> alternate signal stack size, this check is still necessary to protect
> binaries with insufficient alternate signal stack size from data
> corruption.
...
> 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.

	David

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