linux-kernel - Re: [RFC PATCH 00/18] Try to free user PTE page table pages

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Date:   Wed, 18 May 2022 15:56:17 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Qi Zheng <zhengqi.arch@...edance.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        songmuchun@...edance.com, zhouchengming@...edance.com,
        akpm@...ux-foundation.org, tglx@...utronix.de,
        kirill.shutemov@...ux.intel.com, jgg@...dia.com, tj@...nel.org,
        dennis@...nel.org, ming.lei@...hat.com
Subject: Re: [RFC PATCH 00/18] Try to free user PTE page table pages

On Wed, May 18, 2022 at 04:51:06PM +0200, David Hildenbrand wrote:
> yes, I talked about the involved challenges, especially, how malicious
> user space can trigger allocation of almost elusively page tables and
> essentially consume a lot of unmovable+unswappable memory and even store
> secrets in the page table structure.

There are a lot of ways for userspace to consume a large amount of
kernel memory.  For example, one can open a file and set file locks on
alternate bytes.  We generally handle this by accounting the memory to
the process and let the OOM killer, rlimits, memcg or other mechanism
take care of it.  Just because page tables are (generally) reclaimable
doesn't mean we need to treat them specially.