linux-kernel - RE: [PATCH v8 5/6] x86/signal: Detect and prevent an alternate signal stack overflow

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Message-ID: <854d6aefdf604b559e37e82669b5e67f@AcuMS.aculab.com>
Date:   Thu, 22 Apr 2021 08:46:14 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'Chang S. Bae'" <chang.seok.bae@...el.com>,
        "bp@...e.de" <bp@...e.de>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "luto@...nel.org" <luto@...nel.org>,
        "x86@...nel.org" <x86@...nel.org>
CC:     "len.brown@...el.com" <len.brown@...el.com>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>,
        "hjl.tools@...il.com" <hjl.tools@...il.com>,
        "Dave.Martin@....com" <Dave.Martin@....com>,
        "jannh@...gle.com" <jannh@...gle.com>,
        "mpe@...erman.id.au" <mpe@...erman.id.au>,
        "carlos@...hat.com" <carlos@...hat.com>,
        "tony.luck@...el.com" <tony.luck@...el.com>,
        "ravi.v.shankar@...el.com" <ravi.v.shankar@...el.com>,
        "libc-alpha@...rceware.org" <libc-alpha@...rceware.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v8 5/6] x86/signal: Detect and prevent an alternate signal
 stack overflow

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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