linux-kernel - RE: [PATCH v2 3/6] ima: Fix ima digest hash table key calculation

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Date:   Mon, 27 Apr 2020 14:28:24 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Roberto Sassu' <roberto.sassu@...wei.com>,
        "zohar@...ux.ibm.com" <zohar@...ux.ibm.com>,
        "rgoldwyn@...e.de" <rgoldwyn@...e.de>
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>,
        Silviu Vlasceanu <Silviu.Vlasceanu@...wei.com>,
        Krzysztof Struczynski <krzysztof.struczynski@...wei.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH v2 3/6] ima: Fix ima digest hash table key calculation

From: Roberto Sassu
> Sent: 27 April 2020 13:51
...
> > > -static inline unsigned long ima_hash_key(u8 *digest)
> > > +static inline unsigned int ima_hash_key(u8 *digest)
> > >  {
> > > -	return hash_long(*digest, IMA_HASH_BITS);
> > > +	return (*(unsigned int *)digest % IMA_MEASURE_HTABLE_SIZE);
> >
> > That almost certainly isn't right.
> > It falls foul of the *(integer_type *)ptr being almost always wrong.
> 
> I didn't find the problem. Can you please explain?

The general problem with *(int_type *)ptr is that it does completely
the wrong thing if 'ptr' is the address of a larger integer type on
a big-endian system.
You may also get a misaligned access trap.

In this case I guess that digest is actually u8[SHA1_DIGEST_SIZE].
Maybe what you should return is:
	(digest[0] | digest[1] << 8) % IMA_MEASURE_HTABLE_SIZE;
and comment that there is no point taking a hash of part of
a SHA1 digest.

	David

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