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

[Mimi Zohar]

On Fri, 2020-04-24 at 12:18 +0000, Roberto Sassu wrote:

> > 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.
> 
> Before I calculated max depth until there is a match, not the full depth.
> 
> #1
> return hash_long(*((unsigned long *)digest), IMA_HASH_BITS);
> #define IMA_HASH_BITS 9
> 
> Runtime measurements: 1488
> Violations: 0
> Slots (used/available): 484/512
> Max depth hash table: 10
> 
> #2
> return *(unsigned long *)digest % IMA_MEASURE_HTABLE_SIZE;
> #define IMA_HASH_BITS 9
> 
> Runtime measurements: 1491
> Violations: 2
> Slots (used/available): 489/512
> Max depth hash table: 10
> 
> #3
> return hash_long(*((unsigned long *)digest), IMA_HASH_BITS);
> #define IMA_HASH_BITS 10
> 
> Runtime measurements: 1489
> Violations: 0
> Slots (used/available): 780/1024
> Max depth hash table: 6
> 
> #4
> return *(unsigned long *)digest % IMA_MEASURE_HTABLE_SIZE;
> #define IMA_HASH_BITS 10
> 
> Runtime measurements: 1489
> Violations: 0
> Slots (used/available): 793/1024
> Max depth hash table: 6

At least for this measurement list sample, there doesn't seem to be
any benefit to hashing the digest.  In terms of increasing the number
of slots, the additional memory is minimal and shouldn't negatively
affect small embedded devices.  Please make sure checkpatch doesn't
flag it.

thanks,

Mimi