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

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Message-ID: <c998edbed55d4fe78df9a3b224dd0101@huawei.com>
Date:   Tue, 28 Apr 2020 07:19:59 +0000
From:   Roberto Sassu <roberto.sassu@...wei.com>
To:     David Laight <David.Laight@...LAB.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

> -----Original Message-----
> From: David Laight [mailto:David.Laight@...LAB.COM]
> Sent: Monday, April 27, 2020 4:28 PM
> To: Roberto Sassu <roberto.sassu@...wei.com>; zohar@...ux.ibm.com;
> rgoldwyn@...e.de
> Cc: linux-integrity@...r.kernel.org; linux-security-module@...r.kernel.org;
> linux-kernel@...r.kernel.org; Silviu Vlasceanu
> <Silviu.Vlasceanu@...wei.com>; Krzysztof Struczynski
> <krzysztof.struczynski@...wei.com>; 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.

Ok, thanks.

Roberto

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> 
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