linux-kernel - Re: [PATCH 3/5] ima: Fix ima digest hash table key calculation

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Date:   Thu, 23 Apr 2020 12:53:01 -0400
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Roberto Sassu <roberto.sassu@...wei.com>
Cc:     "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>,
        Krzysztof Struczynski <krzysztof.struczynski@...wei.com>,
        Silviu Vlasceanu <Silviu.Vlasceanu@...wei.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH 3/5] ima: Fix ima digest hash table key calculation

On Thu, 2020-04-23 at 10:21 +0000, Roberto Sassu wrote:
> > Hi Roberto, Krsysztof,
> > 
> > On Wed, 2020-03-25 at 17:11 +0100, Roberto Sassu wrote:
> > > From: Krzysztof Struczynski <krzysztof.struczynski@...wei.com>
> > >
> > > Function hash_long() accepts unsigned long, while currently only one byte
> > > is passed from ima_hash_key(), which calculates a key for ima_htable.
> > Use
> > > more bytes to avoid frequent collisions.
> > >
> > > Length of the buffer is not explicitly passed as a function parameter,
> > > because this function expects a digest whose length is greater than the
> > > size of unsigned long.
> > 
> > Somehow I missed the original report of this problem https://lore.kern
> > el.org/patchwork/patch/674684/.  This patch is definitely better, but
> > how many unique keys are actually being used?  Is it anywhere near
> > IMA_MEASURE_HTABLE_SIZE(512)?
> 
> I did a small test (with 1043 measurements):
> 
> slots: 250, max depth: 9 (without the patch)
> slots: 448, max depth: 7 (with the patch)

448 out of 512 slots are used.

> 
> Then, I increased the number of bits to 10:
> 
> slots: 251, max depth: 9 (without the patch)
> slots: 660, max depth: 4 (with the patch)

660 out of 1024 slots are used.

I wonder if there is any benefit to hashing a digest, instead of just
using the first bits. 

> 
> > Do we need a new securityfs entry to display the number used?
> 
> Probably it is useful only if the administrator can decide the number of slots.

The securityfs suggestion was just a means for triggering the above
debugging info you provided.  Could you provide another patch with the
debugging info?

thanks,

Mimi