Content-Type: text/plain; charset=us-ascii
Mime-Version: 1.0 (Mac OS X Mail 10.3 \(3273\))
Subject: Re: [PATCH] mpt3sas: downgrade full copy_from_user to access_ok check
From: Meng Xu <mengxu.gatech@gmail.com>
In-Reply-To: <20170921032604.GF32076@ZenIV.linux.org.uk>
Date: Wed, 20 Sep 2017 23:32:33 -0400
Cc: sathya.prakash@broadcom.com, chaitra.basappa@broadcom.com,
        suganath-prabu.subramani@broadcom.com, jejb@linux.vnet.ibm.com,
        martin.petersen@oracle.com, MPT-FusionLinux.pdl@broadcom.com,
        linux-scsi@vger.kernel.org, linux-kernel@vger.kernel.org,
        meng.xu@gatech.edu, sanidhya@gatech.edu, taesoo@gatech.edu
Content-Transfer-Encoding: 7bit
Message-Id: <F6EA0E6E-A5EB-4096-B80E-6EAA546EFAA0@gmail.com>
References: <1505877071-76996-1-git-send-email-mengxu.gatech@gmail.com>
 <20170921032604.GF32076@ZenIV.linux.org.uk>
To: Al Viro <viro@ZenIV.linux.org.uk>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1072
Lines: 25


> On Sep 20, 2017, at 11:26 PM, Al Viro <viro@ZenIV.linux.org.uk> wrote:
> 
> On Tue, Sep 19, 2017 at 11:11:11PM -0400, Meng Xu wrote:
>> Since right after the user copy, we are going to
>> memset(&karg, 0, sizeof(karg)), I guess an access_ok check is enough?
> 
> access_ok() is *NOT* "will copy_from_user() succeed?"  Not even close.
> On a bunch of architectures (sparc64, for one) access_ok() is always
> true.
> 
> All it does is checking that address is not a kernel one - e.g. on
> i386 anything in range 0..3Gb qualifies.  Whether anything's mapped
> at that address or not.
> 
> Why bother with that copy_from_user() at all?  The same ioctl()
> proceeds to copy_to_user() on exact same range; all you get from
> it is "if the area passed by caller is writable, but not readable,
> fail with -EFAULT".  Who cares?
> 
> Just drop that copy_from_user() completely.  Anything access_ok()
> might've caught will be caught by copy_to_user() anyway.

Yes, Christoph has suggested the same thing and I have submitted 
another patch with copy_from_user removed entirely.