[PATCH 3/5] ima: Fix ima digest hash table key calculation

[Mimi Zohar]

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 at huawei.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