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Date:   Fri, 26 Oct 2018 12:56:56 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     miles.chen@mediatek.com, Matthias Brugger <matthias.bgg@gmail.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        wsd_upstream@mediatek.com, linux-mediatek@lists.infradead.org,
        linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] mm/page_owner: use vmalloc instead of kmalloc
Message-ID: <20181026085410.GA8277@dhcp22.suse.cz>
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 <20181025192701.GK25444@bombadil.infradead.org>
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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