linux-ext4 - Re: [PATCH 10/11] fs: remove the access_ok() check in ioctl

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Date:   Tue, 5 May 2020 12:44:14 +0200
From:   Christoph Hellwig <hch@....de>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     Christoph Hellwig <hch@....de>, linux-ext4@...r.kernel.org,
        viro@...iv.linux.org.uk, jack@...e.cz, tytso@....edu,
        adilger@...ger.ca, riteshh@...ux.ibm.com, amir73il@...il.com,
        linux-fsdevel@...r.kernel.org, linux-unionfs@...r.kernel.org
Subject: Re: [PATCH 10/11] fs: remove the access_ok() check in ioctl_fiemap

On Tue, Apr 28, 2020 at 08:21:24AM -0700, Darrick J. Wong wrote:
> On Mon, Apr 27, 2020 at 08:19:56PM +0200, Christoph Hellwig wrote:
> > access_ok just checks we are fed a proper user pointer.  We also do that
> > in copy_to_user itself, so no need to do this early.
> > 
> > Signed-off-by: Christoph Hellwig <hch@....de>
> 
> Hmm.  It's a minor behavioral change that we no longer require the
> entire extent array to be accessible at the start even if parts of it
> would never have gotten accessed, but I don't think that matters, so:

Note that access_ok only checks if the memory actually is in userspace,
so they only thing seeing a behavior difference would be an exploit of
some kind.