From mboxrd@z Thu Jan 1 00:00:00 1970 From: Al Viro Subject: Re: [PATCH] mpt3sas: downgrade full copy_from_user to access_ok check Date: Thu, 21 Sep 2017 04:26:05 +0100 Message-ID: <20170921032604.GF32076@ZenIV.linux.org.uk> References: <1505877071-76996-1-git-send-email-mengxu.gatech@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <1505877071-76996-1-git-send-email-mengxu.gatech@gmail.com> Sender: linux-kernel-owner@vger.kernel.org To: Meng Xu 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 List-Id: linux-scsi@vger.kernel.org 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.