Re: [PATCH v7 09/14] x86/kprobes: Instruction pages initialization enhancements

[Masami Hiramatsu <mhiramat@kernel.org>]

On Tue, 4 Dec 2018 17:34:03 -0800
Nadav Amit <namit@vmware.com> wrote:

> This patch is a preparatory patch for a following patch that makes
> module allocated pages non-executable. The patch sets the page as
> executable after allocation.
> 
> In the future, we may get better protection of executables. For example,
> by using hypercalls to request the hypervisor to protect VM executable
> pages from modifications using nested page-tables. This would allow
> us to ensure the executable has not changed between allocation and
> its write-protection.

Sounds interesting!

> 
> While at it, do some small cleanup of what appears to be unnecessary
> masking.

Looks good to me.

Acked-by: Masami Hiramatsu <mhiramat@kernel.org>


Thanks!

> 
> Cc: Masami Hiramatsu <mhiramat@kernel.org>
> Signed-off-by: Nadav Amit <namit@vmware.com>
> ---
>  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 c33b06f5faa4..ca0118d3b3e8 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;
> +
> +	/*
> +	 * First make the page read-only, and then only then make it executable
> +	 * 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);
>  }
>  
> -- 
> 2.17.1
> 


-- 
Masami Hiramatsu <mhiramat@kernel.org>