linux-kernel - Re: Copy large memory regions from & to userspace

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Message-id: <46D8C4DC.2080307@shaw.ca>
Date:	Fri, 31 Aug 2007 19:48:12 -0600
From:	Robert Hancock <hancockr@...w.ca>
To:	Clemens Kolbitsch <clemens.kol@....at>
Cc:	Chris Smith <smitty1elkml@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Copy large memory regions from & to userspace

Clemens Kolbitsch wrote:
> On Friday 31 August 2007 15:25:40 you wrote:
>> On 8/30/07, Clemens Kolbitsch <clemens.kol@....at> wrote:
>>> Hi!
>>> Just a short question: What is the correct method of copying large areas
>>> of memory from userspace into userspace when running in kernel-mode?
>> relayfs?
> 
> no... I'm copying user-memory to user-memory, not kernel-to-user, however 
> running the code in kernel-mode.
> 
> what i wanted to know is how to check the access-rights...
> i didn't get any other answers, so for now i'm just using 
> 
> if (access_ok(VERIFY_READ, from, PAGE_SIZE) &&
>     access_ok(VERIFY_WRITE, to, PAGE_SIZE))
> {
>     memcpy(to, from, PAGE_SIZE);
> }
> 
> and hope that this is the *correct* way to do it...

No, it's not. access_ok does not guarantee that the memory region can be 
validly read or written. It only allows using __copy_to_user or 
__copy_from_user which skips the same checks that access_ok does.

I'm not aware of any code in the kernel that does userspace-to-userspace 
copies directly. Likely because there's rarely a need for it?

-- 
Robert Hancock      Saskatoon, SK, Canada
To email, remove "nospam" from hancockr@...pamshaw.ca
Home Page: http://www.roberthancock.com/

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