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SubjectRe: [PATCH] arm64: kprobe: make page to RO mode when allocate it
On Mon, Oct 29, 2018 at 01:11:24PM +0100, Arnd Bergmann wrote:
> On 10/29/18, Will Deacon <will.deacon@arm.com> 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

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