linux-kernel - Re: [PATCH] mpt3sas: downgrade full copy_from_user to access

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Date:   Thu, 21 Sep 2017 04:26:05 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Meng Xu <mengxu.gatech@...il.com>
Cc:     sathya.prakash@...adcom.com, chaitra.basappa@...adcom.com,
        suganath-prabu.subramani@...adcom.com, jejb@...ux.vnet.ibm.com,
        martin.petersen@...cle.com, MPT-FusionLinux.pdl@...adcom.com,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        meng.xu@...ech.edu, sanidhya@...ech.edu, taesoo@...ech.edu
Subject: Re: [PATCH] mpt3sas: downgrade full copy_from_user to access_ok check

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.