Re: [PATCH] mm/page_owner: use vmalloc instead of kmalloc

Michal Hocko
2018-10-26T10:57:01Z

On Thu 25-10-18 12:27:01, Matthew Wilcox wrote:
> On Fri, Oct 26, 2018 at 02:34:41AM +0800, miles.chen@mediatek.com wrote:
> > The kbuf used by page owner is allocated by kmalloc(),
> > which means it can use only normal memory and there might
> > be a "out of memory" issue when we're out of normal memory.
> >
> > Use vmalloc() so we can also allocate kbuf from highmem
> > on 32bit kernel.
>
> ... hang on, there's a bigger problem here.
>
> static const struct file_operations proc_page_owner_operations = {
> .read = read_page_owner,
> };
>
> read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
> {
> ...
> return print_page_owner(buf, count, pfn, page,
> page_owner, handle);
> }
>
> static ssize_t
> print_page_owner(char __user *buf, size_t count, unsigned long pfn,
> struct page *page, struct page_owner *page_owner,
> depot_stack_handle_t handle)
> {
> ...
> kbuf = kmalloc(count, GFP_KERNEL);
>
> So I can force the kernel to make an arbitrary size allocation, triggering
> OOMs and forcing swapping if I can get a file handle to this file.
> The only saving grace is that (a) this is a debugfs file and (b) it's
> root-only (mode 0400). Nevertheless, I feel some clamping is called
> for here. Do we really need to output more than 4kB worth of text here?

Completely agreed. Let's just clamp it to a single page. Userspace can
easily loop around the syscall.

--
Michal Hocko
SUSE Labs