Re: [PATCH tip/x86/asm] uprobes: __create_xol_area() must nullify xol_mapping.fault

[Peter Zijlstra]

On Sat, Feb 27, 2016 at 11:11:28PM +0100, Oleg Nesterov wrote:
> As Jiri pointed out, the recent commit f872f5400cc0 "mm: Add a
> vm_special_mapping.fault() method" breaks uprobes. __create_xol_area()
> doesn't initialize the new ->fault() method and this obviously leads to
> kernel crash when the application tries to execute the probed insn after
> bp hit.
> 
> We probably want to add uprobes_special_mapping_fault(), this allows to
> turn xol_area->xol_mapping into a single instance of vm_special_mapping.
> But we need a simple fix, so lets change __create_xol() to nullify the
> new member as Jiri suggests.
> 
> Reported-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> Suggested-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> ---
>  kernel/events/uprobes.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 0167679..5f6ce93 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -1178,6 +1178,7 @@ static struct xol_area *__create_xol_area(unsigned long vaddr)
>  		goto free_area;
>  
>  	area->xol_mapping.name = "[uprobes]";
> +	area->xol_mapping.fault = NULL;
>  	area->xol_mapping.pages = area->pages;

Would not something like:

	area->xol_mapping = (struct vm_special_mapping){
		.name = "[uprobes]",
		.pages = area->pages,
	};

Be a more robust approach? That way, if someone adds more fields, they
at least get initialized (to 0).

>  	area->pages[0] = alloc_page(GFP_HIGHUSER);
>  	if (!area->pages[0])