linux-kernel - Re: [PATCH 3/3] ima: use fs method to read integrity data

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Date:   Fri, 15 Sep 2017 11:21:21 -0400
From:   Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:     Christoph Hellwig <hch@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Dave Kleikamp <shaggy@...nel.org>,
        Bob Peterson <rpeterso@...hat.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Chao Yu <yuchao0@...wei.com>, Hugh Dickins <hughd@...gle.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Matthew Garrett <mjg59@...f.ucam.org>,
        Joel Becker <jlbec@...lplan.org>, Jan Kara <jack@...e.com>,
        Chris Mason <clm@...com>,
        Ryusuke Konishi <konishi.ryusuke@....ntt.co.jp>,
        Steven Whitehouse <swhiteho@...hat.com>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        "Theodore Ts'o" <tytso@....edu>, Mark Fasheh <mfasheh@...sity.com>,
        linux-security-module@...r.kernel.org,
        linux-ima-devel@...ts.sourceforge.net,
        James Morris <jmorris@...ei.org>,
        Richard Weinberger <richard@....at>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 3/3] ima: use fs method to read integrity data

On Fri, 2017-09-15 at 07:49 -0700, Christoph Hellwig wrote:
> On Thu, Sep 14, 2017 at 10:50:27PM -0700, Linus Torvalds wrote:
> > This is still wrong.
> > 
> > (a) there is no explanation for why we need that exclusive lock in the
> > first place
> > 
> > Why should a read need exclusive access? You'd think shared is sufficient.
> > But regardless, it needs *explanation*.
> 
> Shared is sufficient, and nothing in the patch (except for the
> description) actually requires an exclusive lock.  It just happens that
> ima holds it exclusive for other internal reasons.

Although reading the file to calculate the file hash doesn't require
taking the lock exclusively, in either "fix" mode or called from
__fput, immediately after calculating the file hash, the file hash is
written out as an xattr.  Writing the xattr requires taking the lock
exclusively.

Mimi