Subject: Re: copy_from_user/copy_to_user question
From: Steven Rostedt <rostedt@goodmis.org>
To: Al Viro <viro@ftp.linux.org.uk>
Cc: Vinay Venkataraghavan <raghavanvinay@yahoo.com>,
       linux-kernel@vger.kernel.org
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Date: Fri, 02 Dec 2005 21:02:26 -0500
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On Sat, 2005-12-03 at 01:38 +0000, Al Viro wrote:
> On Fri, Dec 02, 2005 at 08:09:59PM -0500, Steven Rostedt wrote:
> > > Secondly, they seem to use memcpy as opposed to using
> > > copy_to_user/copy_from_user which is also very
> > > dangerous.
> > 
> > If they are grabbing data from user context into kernel (or vise versa)
> > that could easily cause an oops.  Not to mention it is a security risk.
> 
> Not to mention it simply won't work on a many platforms, no matter what...

Hmm, I've only worked with a few platforms (i386, x86_64, ppc, mips, and
a little arm but I don't remember that much).  I believe that a memcpy
could work on all these platforms (error prone of course, but if the
memory is mapped its OK).  When entering a system call, the kernel still
has access to the memory locations assigned to the user.

It's been a while since I had to deal with the semantics of
copy_to/from_user.  So what platforms can not access a user space area
directly.  Is there a special assembly command to use to copy from user?

I haven't dealt (yet) with the copy_user of x86_64.  Is there a problem
when one tries to copy to/from a 32 bit address while in a 64 bit
address space?

Just curious, sorry for my ignorance here.

-- Steve
 

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