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

[Roberto Sassu]

> -----Original Message-----
> From: David Laight [mailto:David.Laight@xxxxxxxxxx]
> Sent: Monday, April 27, 2020 4:28 PM
> To: Roberto Sassu <roberto.sassu@xxxxxxxxxx>; zohar@xxxxxxxxxxxxx;
> rgoldwyn@xxxxxxx
> Cc: linux-integrity@xxxxxxxxxxxxxxx; linux-security-module@xxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; Silviu Vlasceanu
> <Silviu.Vlasceanu@xxxxxxxxxx>; Krzysztof Struczynski
> <krzysztof.struczynski@xxxxxxxxxx>; stable@xxxxxxxxxxxxxxx
> 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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