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

[David Daney]

On 11/21/2017 05:56 AM, Aleksandar Markovic wrote:
> From: Miodrag Dinic <miodrag.dinic@xxxxxxxx>
>
> 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@xxxxxxxx>
> Signed-off-by: Aleksandar Markovic <aleksandar.markovic@xxxxxxxx>
> ---
>   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;