linux-kernel - Re: [PATCH] LinuxPPS (with new syscalls API)

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Date:	Tue, 26 Jun 2007 17:33:28 -0600
From:	Robert Hancock <hancockr@...w.ca>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	Rodolfo Giometti <giometti@...eenne.com>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] LinuxPPS (with new syscalls API)

David Woodhouse wrote:
> On Tue, 2007-06-26 at 19:06 +0200, Rodolfo Giometti wrote:
>> On Tue, Jun 26, 2007 at 11:57:07AM +0100, David Woodhouse wrote:
>>> Your syscalls blindly dereference userspace pointers instead of using
>>> copy_{to,from} user.
>> I use access_ok() to test userspace addresses. It should be ok,
>> shouldn't it?
> 
> No; it's racy. You must use copy_from_user() and copy_to_user().

Not only is it racy, but it doesn't even do all of the checks that 
copy_to/from_user does. access_ok only validates that the region given 
is potentially valid, not that it actually is. Using access_ok only 
allows you to use __copy_to/from_user instead, which skips the same 
checks that access_ok does - not worth it unless you do repeated copies 
to/from the same region of memory.

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

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