linux-kernel - Re: [PATCH] fs: Return raw xattr for security.* if there is size disagreement with LSMs

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Date:   Fri, 18 Jun 2021 12:35:22 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Mimi Zohar <zohar@...ux.ibm.com>
Cc:     Roberto Sassu <roberto.sassu@...wei.com>,
        Stefan Berger <stefanb@...ux.ibm.com>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "stephen.smalley.work@...il.com" <stephen.smalley.work@...il.com>,
        "casey@...aufler-ca.com" <casey@...aufler-ca.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "selinux@...r.kernel.org" <selinux@...r.kernel.org>
Subject: Re: [PATCH] fs: Return raw xattr for security.* if there is size
 disagreement with LSMs

On Fri, Jun 18, 2021 at 12:04 PM Mimi Zohar <zohar@...ux.ibm.com> wrote:
> On Thu, 2021-06-17 at 23:18 -0400, Paul Moore wrote:
> > On Thu, Jun 17, 2021 at 11:28 AM Mimi Zohar <zohar@...ux.ibm.com> wrote:
> > > On Thu, 2021-06-17 at 07:09 +0000, Roberto Sassu wrote:
> >
> > ...
> >
> > > > An alternative would be to do the EVM verification twice if the
> > > > first time didn't succeed (with vfs_getxattr_alloc() and with the
> > > > new function that behaves like vfs_getxattr()).
> > >
> > > Unfortunately, I don't see an alternative.
> >
> > ... and while unfortunate, the impact should be non-existant if you
> > are using the right tools to label files or ensuring that you are
> > formatting labels properly if doing it by hand.
> >
> > Handling a corner case is good, but I wouldn't add a lot of code
> > complexity trying to optimize it.
>
> From userspace it's really difficult to understand the EVM signature
> verification failure is due to the missing NULL.

I would argue that any signature verification failure, regardless of
the mechanism, is hard to understand.  It either passes or it fails,
and if it fails good luck trying to determine what exactly isn't
matching up; especially if you really don't know the Right Value.

What I mean by the corner case was the fact that the recommended tools
should always do the right thing with respect to '\0' termination,
this should really only be an issue if someone is winging it and doing
it by hand or with their own tools.

-- 
paul moore
www.paul-moore.com