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Subject: Re: copy_from_user/copy_to_user question
Date: Mon, 5 Dec 2005 08:31:19 -0500
Message-ID: <Pine.LNX.4.61.0512050820390.27133@chaos.analogic.com>
Thread-Topic: copy_from_user/copy_to_user question
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From: "linux-os \(Dick Johnson\)" <linux-os@analogic.com>
To: "Steven Rostedt" <rostedt@goodmis.org>
Cc: "Vinay Venkataraghavan" <raghavanvinay@yahoo.com>,
       <linux-kernel@vger.kernel.org>
Reply-To: "linux-os \(Dick Johnson\)" <linux-os@analogic.com>
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On Fri, 2 Dec 2005, Steven Rostedt wrote:

> On Fri, 2005-12-02 at 14:40 -0800, Vinay Venkataraghavan wrote:
>> I have a question regarding copy_to_user and
>> copy_from_user, specifically the conditons and
>> situations when they can be used.
>>
>> Firstly, I guess it is always safe to use these
>> funtions when making an ioctl call.
>
> It's basically safe whenever you can schedule, and you are running on
> behalf of a user task (as appose to a kernel thread).
>
>>
>> But my question is: Are there any specific
>> circumstances or conditions when these functions don't
>> have to be used, but at the same time ensure that no
>> page fault occurs and crashes the system.
>
> Sure, they don't need to be used if you don't need to get data to or
> from user context.
>
>>
>> The reason I ask is, there is some software that I am
>> dealing with that just don't use these functions.
>
> What is this code and what is it doing?
>
>>
>> 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.
>
> -- Steve
>

The kernel has the privileges to trash anything from kernel-space.
This means that an incorrect pointer or length-variable, passed
from user-mode code could trash everything including the kernel
if the kernel code just used memcpy().

So, copy_to/from_user was developed so that the user's privs and
page ownership would be used during the copy operation. This means
that if the user provides the wrong location or byte-count, the
improper operation is trapped and the kernel code can return
an error code, usually -EFAULT.

Drivers that use memcpy() when accessing user-provided space
(such as in read(), write(), ioctl(), etc.) are simply broken
and should not be used. They can cause other modules and/or
kernel functions to fail in mysterious ways.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.13.4 on an i686 machine (5589.44 BogoMips).
Warning : 98.36% of all statistics are fiction.
.

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