linux-kernel - Re: [PATCH v2] MIPS: Add nonxstack=on|off kernel parameter

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Message-ID: <dda5572e-0617-3427-7a90-07b3cf43d808@caviumnetworks.com>
Date:   Tue, 21 Nov 2017 12:53:17 -0800
From:   David Daney <ddaney@...iumnetworks.com>
To:     Aleksandar Markovic <aleksandar.markovic@...rk.com>,
        linux-mips@...ux-mips.org
Cc:     Miodrag Dinic <miodrag.dinic@...s.com>,
        Aleksandar Markovic <aleksandar.markovic@...s.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dengcheng Zhu <dengcheng.zhu@...s.com>,
        Ding Tianhong <dingtianhong@...wei.com>,
        Douglas Leung <douglas.leung@...s.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Goran Ferenc <goran.ferenc@...s.com>,
        Ingo Molnar <mingo@...nel.org>,
        James Cowgill <James.Cowgill@...tec.com>,
        James Hogan <james.hogan@...s.com>,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, Marc Zyngier <marc.zyngier@....com>,
        Matt Redfearn <matt.redfearn@...s.com>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Paul Burton <paul.burton@...s.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Petar Jovanovic <petar.jovanovic@...s.com>,
        Raghu Gandham <raghu.gandham@...s.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tom Saeger <tom.saeger@...cle.com>
Subject: Re: [PATCH v2] MIPS: Add nonxstack=on|off kernel parameter

On 11/21/2017 05:56 AM, Aleksandar Markovic wrote:
> From: Miodrag Dinic <miodrag.dinic@...s.com>
> 
> Add a new kernel parameter to override the default behavior related
> to the decision whether to set up stack as non-executable in function
> mips_elf_read_implies_exec().
> 
> The new parameter is used to control non executable stack and heap,
> regardless of PT_GNU_STACK entry. This does apply to both stack and
> heap, despite the name.
> 
> Allowed values:
> 
> nonxstack=on	Force non-exec stack & heap
> nonxstack=off	Force executable stack & heap
> 
> If this parameter is omitted, kernel behavior remains the same as it
> was before this patch is applied.

Do other architectures have a similar hack?

If arm{,64} and x86 don't need this, what would make MIPS so special 
that we have to carry this around?


> 
> This functionality is convenient during debugging and is especially
> useful for Android development where non-exec stack is required.

Why not just set the PT_GNU_STACK flags correctly in the first place?

> 
> Signed-off-by: Miodrag Dinic <miodrag.dinic@...s.com>
> Signed-off-by: Aleksandar Markovic <aleksandar.markovic@...s.com>
> ---
>   Documentation/admin-guide/kernel-parameters.txt | 11 +++++++
>   arch/mips/kernel/elf.c                          | 39 +++++++++++++++++++++++++
>   2 files changed, 50 insertions(+)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index b74e133..99464ee 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -2614,6 +2614,17 @@
>   			noexec32=off: disable non-executable mappings
>   				read implies executable mappings
>   
> +	nonxstack	[MIPS]
> +			Force setting up stack and heap as non-executable or
> +			executable regardless of PT_GNU_STACK entry. Both
> +			stack and heap are affected, despite the name. Valid
> +			arguments: on, off.
> +			nonxstack=on:	Force non-executable stack and heap
> +			nonxstack=off:	Force executable stack and heap
> +			If ommited, stack and heap will or will not be set
> +			up as non-executable depending on PT_GNU_STACK
> +			entry and possibly other factors.
> +
>   	nofpu		[MIPS,SH] Disable hardware FPU at boot time.
>   
>   	nofxsr		[BUGS=X86-32] Disables x86 floating point extended
> diff --git a/arch/mips/kernel/elf.c b/arch/mips/kernel/elf.c
> index 731325a..28ef7f3 100644
> --- a/arch/mips/kernel/elf.c
> +++ b/arch/mips/kernel/elf.c
> @@ -326,8 +326,47 @@ void mips_set_personality_nan(struct arch_elf_state *state)
>   	}
>   }
>   
> +static int nonxstack = EXSTACK_DEFAULT;
> +
> +/*
> + * kernel parameter: nonxstack=on|off
> + *
> + *   Force setting up stack and heap as non-executable or
> + *   executable regardless of PT_GNU_STACK entry. Both
> + *   stack and heap are affected, despite the name. Valid
> + *   arguments: on, off.
> + *
> + *     nonxstack=on:   Force non-executable stack and heap
> + *     nonxstack=off:  Force executable stack and heap
> + *
> + *   If ommited, stack and heap will or will not be set
> + *   up as non-executable depending on PT_GNU_STACK
> + *   entry and possibly other factors.
> + */
> +static int __init nonxstack_setup(char *str)
> +{
> +	if (!strcmp(str, "on"))
> +		nonxstack = EXSTACK_DISABLE_X;
> +	else if (!strcmp(str, "off"))
> +		nonxstack = EXSTACK_ENABLE_X;
> +	else
> +		pr_err("Malformed nonxstack format! nonxstack=on|off\n");
> +
> +	return 1;
> +}
> +__setup("nonxstack=", nonxstack_setup);
> +
>   int mips_elf_read_implies_exec(void *elf_ex, int exstack)
>   {
> +	switch (nonxstack) {
> +	case EXSTACK_DISABLE_X:
> +		return 0;
> +	case EXSTACK_ENABLE_X:
> +		return 1;
> +	default:
> +		break;
> +	}
> +
>   	if (exstack != EXSTACK_DISABLE_X) {
>   		/* The binary doesn't request a non-executable stack */
>   		return 1;
>