netdev - Re: [RFC] situation with csum_and_copy

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Message-Id: <20141119.153136.867017618826698045.davem@davemloft.net>
Date:	Wed, 19 Nov 2014 15:31:36 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	viro@...IV.linux.org.uk
Cc:	torvalds@...ux-foundation.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] situation with csum_and_copy_... API

From: Al Viro <viro@...IV.linux.org.uk>
Date: Tue, 18 Nov 2014 21:23:07 +0000

> On Tue, Nov 18, 2014 at 12:49:13PM -0800, Linus Torvalds wrote:
>> "access_ok()" isn't that expensive, and removing them as unnecessary
>> is fraught with errors. We've had several cases of "oops, we used
>> __get_user() in a loop, because it generates much better code, but
>> we'd forgotten to do access_ok(), so now people can read kernel data".
> 
> OK...  If netdev folks can live with that for now, I've no problem with
> dropping 3/5.  However, I really think we need a variant of csum-and-copy
> that would _not_ bother with access_ok() longer term.  That can wait, though...

I think because of the way Al verifies things at the top level, and
how we structure access to these msg->msg_iov so strictly, these cases
of access_ok() really can safely go.

But that is just my opinion, and yes I do acknowledge that we've had
serious holes in this area in the past.
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