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

[Matthew Wilcox <willy@infradead.org>]

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?