linux-kernel - Re: [PATCH 3/3] ima: use fs method to read integrity data (updated patch description)

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Date:   Sun, 17 Sep 2017 17:38:28 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Christoph Hellwig <hch@...radead.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        Christoph Hellwig <hch@....de>,
        linux-ima-devel@...ts.sourceforge.net,
        James Morris <jmorris@...ei.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Matthew Garrett <mjg59@...f.ucam.org>,
        Jan Kara <jack@...e.com>, Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <yuchao0@...wei.com>,
        Steven Whitehouse <swhiteho@...hat.com>,
        Bob Peterson <rpeterso@...hat.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Dave Kleikamp <shaggy@...nel.org>,
        Ryusuke Konishi <konishi.ryusuke@....ntt.co.jp>,
        Mark Fasheh <mfasheh@...sity.com>,
        Joel Becker <jlbec@...lplan.org>,
        Richard Weinberger <richard@....at>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Hugh Dickins <hughd@...gle.com>, Chris Mason <clm@...com>
Subject: Re: [PATCH 3/3] ima: use fs method to read integrity data (updated
 patch description)

On Sun, Sep 17, 2017 at 09:34:01AM -0700, Linus Torvalds wrote:
> Now, I suspect most (all?) do, but that's a historical artifact rather
> than "design". In particular, the VFS layer used to do the locking for
> the filesystems, to guarantee the POSIX requirements (POSIX requires
> that writes be seen atomically).
> 
> But that lock was pushed down into the filesystems, since some
> filesystems really wanted to have parallel writes (particularly for
> direct IO, where that POSIX serialization requirement doesn't exist).
> 
> That's all many years ago, though. New filesystems are likely to have
> copied the pattern from old ones, but even then..
> 
> Also, it's worth noting that "inode->i_rwlock" isn't even well-defined
> as a lock. You can have the question of *which* inode gets talked
> about when you have things like eoverlayfs etc. Normally it would be
> obvious, but sometimes you'd use "file->f_mapping->host" (which is the
> same thing in the simple cases), and sometimes it really wouldn't be
> obvious at all..
> 
> So... I'm really not at all convinced that i_rwsem is sensible. It's
> one of those things that are "mostly right for the simple cases",
> but...

The thing pretty much common to all of them is that write() might need
to modify permissions (suid removal), which brings ->i_rwsem in one
way or another - notify_change() needs that held...