linux-kernel - Re: [PATCH] ima: drop vla in ima_audit

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Message-ID: <20180308194746.r2b5jwfqpq2pah35@smitten>
Date:   Thu, 8 Mar 2018 12:47:46 -0700
From:   Tycho Andersen <tycho@...ho.ws>
To:     Mimi Zohar <zohar@...ux.vnet.ibm.com>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
        linux-integrity@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kernel-hardening@...ts.openwall.com,
        Richard Guy Briggs <rgb@...hat.com>
Subject: Re: [PATCH] ima: drop vla in ima_audit_measurement()

On Thu, Mar 08, 2018 at 02:20:17PM -0500, Mimi Zohar wrote:
> 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@...ho.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?

It's just uppercase and lowercase in the hash value, so:

Mar  8 16:56:46 ima kernel: [  104.922927] audit: type=1805 audit(1520528206.082:53): file="/bin/cat" hash="sha1:79e52322102f073684e2dd0ab7653c7c6fcc49b4" ppid=2049 pid=2123 auid=1000 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=1 comm="bash" exe="/bin/bash"

vs.

Mar  8 19:45:12 ima kernel: [  207.124383] audit: type=1805 audit(1520538312.740:239): file="/root/.viminfo" hash="sha1:3322BE0C00190AB0D20C47574575842EC3020BF5" ppid=2045 pid=2195 auid=1000 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=1 comm="vi" exe="/usr/bin/vim.basic"

I'm happy to do either way.

Tycho