Re: [PATCH v2 09/20] x86/kprobes: instruction pages initialization enhancements

[Borislav Petkov]

Only nitpicks:

> Subject: Re: [PATCH v2 09/20] x86/kprobes: instruction pages initialization enhancements

Subject needs a verb.

On Mon, Jan 28, 2019 at 04:34:11PM -0800, Rick Edgecombe wrote:
> From: Nadav Amit <namit@xxxxxxxxxx>
> 
> Make kprobes instruction pages read-only (and executable) after they are
> set to prevent them from mistaken or malicious modifications.
> 
> This is a preparatory patch for a following patch that makes module
> allocated pages non-executable and sets the page as executable after
> allocation.
> 
> While at it, do some small cleanup of what appears to be unnecessary
> masking.
> 
> Acked-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> ---
>  arch/x86/kernel/kprobes/core.c | 24 ++++++++++++++++++++----
>  1 file changed, 20 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> index 4ba75afba527..fac692e36833 100644
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -431,8 +431,20 @@ void *alloc_insn_page(void)
>  	void *page;
>  
>  	page = module_alloc(PAGE_SIZE);
> -	if (page)
> -		set_memory_ro((unsigned long)page & PAGE_MASK, 1);
> +	if (page == NULL)
> +		return NULL;

Null tests we generally do like this:

	if (! ...


like in the rest of this file.

> +
> +	/*
> +	 * First make the page read-only, and then only then make it executable

 s/then only then/only then/

ditto below.

> +	 * to prevent it from being W+X in between.
> +	 */
> +	set_memory_ro((unsigned long)page, 1);
> +
> +	/*
> +	 * TODO: Once additional kernel code protection mechanisms are set, ensure
> +	 * that the page was not maliciously altered and it is still zeroed.
> +	 */
> +	set_memory_x((unsigned long)page, 1);
>  
>  	return page;
>  }
> @@ -440,8 +452,12 @@ void *alloc_insn_page(void)
>  /* Recover page to RW mode before releasing it */
>  void free_insn_page(void *page)
>  {
> -	set_memory_nx((unsigned long)page & PAGE_MASK, 1);
> -	set_memory_rw((unsigned long)page & PAGE_MASK, 1);
> +	/*
> +	 * First make the page non-executable, and then only then make it
> +	 * writable to prevent it from being W+X in between.
> +	 */
> +	set_memory_nx((unsigned long)page, 1);
> +	set_memory_rw((unsigned long)page, 1);
>  	module_memfree(page);
>  }
>  
> -- 

-- 
Regards/Gruss,
    Boris.

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