linux-hardening - Re: [PATCH -next] mm: usercopy: add a debugfs interface to bypass the vmalloc check.

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Message-ID: <Z1AX-xhOZEZqH1Jw@pc636>
Date: Wed, 4 Dec 2024 09:51:07 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Uladzislau Rezki <urezki@...il.com>,
	Kefeng Wang <wangkefeng.wang@...wei.com>, zuoze <zuoze1@...wei.com>,
	gustavoars@...nel.org, akpm@...ux-foundation.org,
	linux-hardening@...r.kernel.org, linux-mm@...ck.org,
	keescook@...omium.org
Subject: Re: [PATCH -next] mm: usercopy: add a debugfs interface to bypass
 the vmalloc check.

> On Tue, Dec 03, 2024 at 08:02:26PM +0100, Uladzislau Rezki wrote:
> 
> I think there are a few other things we can try here.
> 
> First, if the copy is small (and I still don't have an answer to that
> ...), we can skip the vmalloc lookup if the copy doesn't cross a page
> boundary.
>
I noticed that, a path which is in question, does not supply a starting
address of mapping area, instead it passes something within.

> 
> Second, we could try storing this in a maple tree rather than an rbtree.
> That gives us RCU protected lookups rather than under a spinlock.
> 
I think, when i have more free cycles, i will check it from performance
point of view. Because i do not know how much a maple tree is efficient
when it comes to lookups, insert and removing.

As an RCU safe data structure, yes, a searching is improved in a way there
is no need in taking spinlock. As a noted earlier i do not know if a maple 
tree allows to find a data when instead of key, it is associated with, we
pass something that is withing a searchable area: [va_start:va_end].

--
Uladzislau Rezki