Re: [PATCH] ima: drop vla in ima_audit_measurement()

From: Mimi Zohar <zohar@linux.vnet.ibm.com>
To: Tycho Andersen <tycho@tycho.ws>
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>,
	Dmitry Kasatkin <dmitry.kasatkin@gmail.com>,
	linux-integrity@vger.kernel.org,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	kernel-hardening@lists.openwall.com,
	Richard Guy Briggs <rgb@redhat.com>
Subject: Re: [PATCH] ima: drop vla in ima_audit_measurement()
Date: Thu, 08 Mar 2018 14:20:17 -0500	[thread overview]
Message-ID: <1520536817.3605.74.camel@linux.vnet.ibm.com> (raw)
In-Reply-To: <20180308190431.5fstqsjmbpquzqii@smitten>

On Thu, 2018-03-08 at 12:04 -0700, Tycho Andersen wrote:
> On Thu, Mar 08, 2018 at 01:50:30PM -0500, Mimi Zohar wrote:
> > On Thu, 2018-03-08 at 11:37 -0700, Tycho Andersen wrote:
> > > On Thu, Mar 08, 2018 at 07:47:37PM +0200, Andy Shevchenko wrote:
> > > > On Thu, Mar 8, 2018 at 7:14 PM, Tycho Andersen <tycho@tycho.ws> wrote:
> > > > > In keeping with the directive to get rid of VLAs [1], let's drop the VLA
> > > > > from ima_audit_measurement(). We need to adjust the return type of
> > > > > ima_audit_measurement, because now this function can fail if an allocation
> > > > > fails.
> > > > 
> > > > 
> > > > 
> > > > > +       algo_hash_len = hash_len + strlen(algo_name) + 2;
> > > > > +       algo_hash = kzalloc(algo_hash_len, GFP_KERNEL);
> > > > 
> > > > > -       snprintf(algo_hash, sizeof(algo_hash), "%s:%s", algo_name, hash);
> > > > > +       snprintf(algo_hash, algo_hash_len, "%s:%s", algo_name, hash);
> > > > 
> > > > kasprintf() ?
> > > 
> > > Sure, in fact I think we could just do:
> > > 
> > > -	snprintf(algo_hash, algo_hash_len, "%s:%s", algo_name, hash);
> > > -	audit_log_untrustedstring(ab, algo_hash);
> > > +	audit_log_untrustedstring(ab, algo_name);
> > > +	audit_log_format(ab, ":");
> > > +	audit_log_untrustedstring(ab, hash);
> > > 
> > > and get rid of the allocation entirely. I'll test and make sure it
> > > works and then re-send.
> > 
> > The hash algorithm name is an enumeration that comes from the kernel.
> >  It's defined in crypto/hash_info.c: hash_algo_name.  Why do we need
> > to use audit_log_untrustedstring()?
> 
> Yes, I suppose we don't need it for the hash either, since we're
> generating that and we know it's just hex digits and not any audit
> control characters or "s or anything.
> 
> It looks like we could get rid of the other allocation too by just
> using audit_log_n_hex, but that uses hex_byte_pack_upper, vs. the
> hex_byte_pack that's currently in use in this function. Is that too
> much of a breakage?

Based on the discussion with Richard Briggs, we need to differentiate
between the ima_audit_measurement() and the ima_parse_rule() usage of
AUDIT_INTEGRITY_RULE.  The ima_parse_rule() will continue to use
AUDIT_INTEGRITY_RULE.  ima_audit_measurement() will need to define and
use a new number.  Auidt name suggestions would be appreciated.

When we make that sort of change, any other changes are insignificant.
How different are the two formats?

Mimi