linux-kernel - Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been taken exclusively

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Message-Id: <1506947109.5691.282.camel@linux.vnet.ibm.com>
Date:   Mon, 02 Oct 2017 08:25:09 -0400
From:   Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Dave Chinner <david@...morbit.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Christoph Hellwig <hch@...radead.org>,
        "Theodore Ts'o" <tytso@....edu>, Jan Kara <jack@...e.cz>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-integrity@...r.kernel.org,
        Sascha Hauer <s.hauer@...gutronix.de>
Subject: Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been
 taken exclusively

On Sun, 2017-10-01 at 22:25 -0500, Eric W. Biederman wrote:
> Mimi Zohar <zohar@...ux.vnet.ibm.com> writes:

> > There should be no open writers in ima_check_last_writer(), so the
> > file shouldn't be changing.
> 
> This is slightly tangential but I think important to consider.
> What do you do about distributed filesystems fuse, nfs, etc that
> can change the data behind the kernels back.

Exactly!

> Do you not support such systems or do you have a sufficient way to
> detect changes?

Currently, only the initial file access in policy is measured,
verified, audited.  Even if there was a way of detecting the change,
since we can't trust these file systems, the performance would be
awful, but we should probably not be caching the
measurement/verification results.

Mimi