linux-kernel - Re: [PATCH v3 09/13] powerpc: extend execmem

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Date:   Fri, 22 Sep 2023 10:32:46 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Mike Rapoport <rppt@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
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Subject: Re: [PATCH v3 09/13] powerpc: extend execmem_params for kprobes
 allocations

Hi Mike,

Le 18/09/2023 à 09:29, Mike Rapoport a écrit :
> From: "Mike Rapoport (IBM)" <rppt@...nel.org>
> 
> powerpc overrides kprobes::alloc_insn_page() to remove writable
> permissions when STRICT_MODULE_RWX is on.
> 
> Add definition of EXECMEM_KRPOBES to execmem_params to allow using the
> generic kprobes::alloc_insn_page() with the desired permissions.
> 
> As powerpc uses breakpoint instructions to inject kprobes, it does not
> need to constrain kprobe allocations to the modules area and can use the
> entire vmalloc address space.

I don't understand what you mean here. Does it mean kprobe allocation 
doesn't need to be executable ? I don't think so based on the pgprot you 
set.

On powerpc book3s/32, vmalloc space is not executable. Only modules 
space is executable. X/NX cannot be set on a per page basis, it can only 
be set on a 256 Mbytes segment basis.

See commit c49643319715 ("powerpc/32s: Only leave NX unset on segments 
used for modules") and 6ca055322da8 ("powerpc/32s: Use dedicated segment 
for modules with STRICT_KERNEL_RWX") and 7bee31ad8e2f ("powerpc/32s: Fix 
is_module_segment() when MODULES_VADDR is defined").

So if your intention is still to have an executable kprobes, then you 
can't use vmalloc address space.

Christophe

> 
> Signed-off-by: Mike Rapoport (IBM) <rppt@...nel.org>
> ---
>   arch/powerpc/kernel/kprobes.c | 14 --------------
>   arch/powerpc/kernel/module.c  | 11 +++++++++++
>   2 files changed, 11 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
> index 62228c7072a2..14c5ddec3056 100644
> --- a/arch/powerpc/kernel/kprobes.c
> +++ b/arch/powerpc/kernel/kprobes.c
> @@ -126,20 +126,6 @@ kprobe_opcode_t *arch_adjust_kprobe_addr(unsigned long addr, unsigned long offse
>   	return (kprobe_opcode_t *)(addr + offset);
>   }
>   
> -void *alloc_insn_page(void)
> -{
> -	void *page;
> -
> -	page = execmem_text_alloc(EXECMEM_KPROBES, PAGE_SIZE);
> -	if (!page)
> -		return NULL;
> -
> -	if (strict_module_rwx_enabled())
> -		set_memory_rox((unsigned long)page, 1);
> -
> -	return page;
> -}
> -
>   int arch_prepare_kprobe(struct kprobe *p)
>   {
>   	int ret = 0;
> diff --git a/arch/powerpc/kernel/module.c b/arch/powerpc/kernel/module.c
> index 824d9541a310..bf2c62aef628 100644
> --- a/arch/powerpc/kernel/module.c
> +++ b/arch/powerpc/kernel/module.c
> @@ -95,6 +95,9 @@ static struct execmem_params execmem_params __ro_after_init = {
>   		[EXECMEM_DEFAULT] = {
>   			.alignment = 1,
>   		},
> +		[EXECMEM_KPROBES] = {
> +			.alignment = 1,
> +		},
>   		[EXECMEM_MODULE_DATA] = {
>   			.alignment = 1,
>   		},
> @@ -135,5 +138,13 @@ struct execmem_params __init *execmem_arch_params(void)
>   
>   	range->pgprot = prot;
>   
> +	execmem_params.ranges[EXECMEM_KPROBES].start = VMALLOC_START;
> +	execmem_params.ranges[EXECMEM_KPROBES].start = VMALLOC_END;
> +
> +	if (strict_module_rwx_enabled())
> +		execmem_params.ranges[EXECMEM_KPROBES].pgprot = PAGE_KERNEL_ROX;
> +	else
> +		execmem_params.ranges[EXECMEM_KPROBES].pgprot = PAGE_KERNEL_EXEC;
> +
>   	return &execmem_params;
>   }