Re: [PATCH 3/4] arm64/kprobes: set VM_FLUSH_RESET_PERMS on kprobe instruction pages

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 05/23/2019 03:52 PM, Ard Biesheuvel wrote:
> In order to avoid transient inconsistencies where freed code pages
> are remapped writable while stale TLB entries still exist on other
> cores, mark the kprobes text pages with the VM_FLUSH_RESET_PERMS
> attribute. This instructs the core vmalloc code not to defer the
> TLB flush when this region is unmapped and returned to the page
> allocator.

Makes sense.

> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
> ---
>  arch/arm64/kernel/probes/kprobes.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
> index 2509fcb6d404..036cfbf9682a 100644
> --- a/arch/arm64/kernel/probes/kprobes.c
> +++ b/arch/arm64/kernel/probes/kprobes.c
> @@ -131,8 +131,10 @@ void *alloc_insn_page(void)
>  	void *page;
>  
>  	page = vmalloc_exec(PAGE_SIZE);
> -	if (page)
> +	if (page) {
>  		set_memory_ro((unsigned long)page, 1);
> +		set_vm_flush_reset_perms(page);
> +	}

Looks good. It seems there might be more users who would like to set
VM_FLUSH_RESET_PERMS right after their allocation for the same reason.
Hence would not it help to have a variant like vmalloc_exec_reset() or
such which will tag vm_struct->flags with VM_FLUSH_RESET_PERMS right
after it's allocation without requiring the caller to do the same.