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

From: Mimi Zohar <zohar@linux.ibm.com>
To: Paul Moore <paul@paul-moore.com>
Cc: Roberto Sassu <roberto.sassu@huawei.com>,
	Stefan Berger <stefanb@linux.ibm.com>,
	"viro@zeniv.linux.org.uk" <viro@zeniv.linux.org.uk>,
	"stephen.smalley.work@gmail.com" <stephen.smalley.work@gmail.com>,
	"casey@schaufler-ca.com" <casey@schaufler-ca.com>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	"linux-integrity@vger.kernel.org"
	<linux-integrity@vger.kernel.org>,
	"linux-security-module@vger.kernel.org" 
	<linux-security-module@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"selinux@vger.kernel.org" <selinux@vger.kernel.org>
Subject: Re: [PATCH] fs: Return raw xattr for security.* if there is size disagreement with LSMs
Date: Fri, 18 Jun 2021 12:04:21 -0400	[thread overview]
Message-ID: <c92d0ac71a8db8bb016a7e94b83c193956d71a26.camel@linux.ibm.com> (raw)
In-Reply-To: <CAHC9VhTv6Zn8gYaB6cG4wPzy_Ty0XjOM-QL4cZ525RnhFY4bTQ@mail.gmail.com>

On Thu, 2021-06-17 at 23:18 -0400, Paul Moore wrote:
> On Thu, Jun 17, 2021 at 11:28 AM Mimi Zohar <zohar@linux.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.

Roberto, I just pushed the "evm: output EVM digest calculation info"
patch to the next-integrity-testing branch, which includes some
debugging.   Instead of this patch, which returns the raw xattr data,
how about adding additional debugging info in evm_calc_hmac_or_hash()
indicating the size discrepancy between the raw xattr and the LSM
returned xattr?

thanks,

Mimi