Re: [PATCH] arm64: kprobe: make page to RO mode when allocate it

[Will Deacon]

On Mon, Oct 29, 2018 at 01:11:24PM +0100, Arnd Bergmann wrote:
> On 10/29/18, Will Deacon <will.deacon@xxxxxxx> wrote:
> > On Mon, Oct 15, 2018 at 01:16:00PM +0200, Anders Roxell wrote:
> 
> >> -static int __kprobes patch_text(kprobe_opcode_t *addr, u32 opcode)
> >> +void *alloc_insn_page(void)
> >>  {
> >> -	void *addrs[1];
> >> -	u32 insns[1];
> >> +	void *page;
> >>
> >> -	addrs[0] = (void *)addr;
> >> -	insns[0] = (u32)opcode;
> >> +	page = vmalloc_exec(PAGE_SIZE);
> >> +	if (page)
> >> +		set_memory_ro((unsigned long)page & PAGE_MASK, 1);
> >
> > This looks a bit strange to me -- you're allocating PAGE_SIZE bytes so
> > that we can adjust the permissions, yet we can't guarantee that page is
> > actually page-aligned and therefore end up explicitly masking down.
> >
> > In which case allocating an entire page isn't actually helping us, and
> > we could end up racing with somebody else changing permission on the
> > same page afaict.
> >
> > I think we need to ensure we really have an entire page, perhaps using
> > vmap() instead? Or have I missed some subtle detail here?
> 
> I'm fairly sure that vmalloc() and vmalloc_exec() is guaranteed to be page
> aligned everywhere. The documentation is a bit vague here, but I'm
> still confident enough that we can make that assumption based on
> 
> /**
>  *      vmalloc_exec  -  allocate virtually contiguous, executable memory
>  *      @size:          allocation size
>  *
>  *      Kernel-internal function to allocate enough pages to cover @size
>  *      the page level allocator and map them into contiguous and
>  *      executable kernel virtual space.
>  *
>  *      For tight control over page level allocator and protection flags
>  *      use __vmalloc() instead.
>  */
> void *vmalloc_exec(unsigned long size)

FWIW, I did a bit of digging and I agree with your conclusion. vmalloc()
allocations end up getting installed in map_vm_area() via
__vmalloc_area_node(), which allocates things a page at a time.

So we can simplify this patch to drop the masking when calling
set_memory_ro().

Will