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 1:00 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 11:29
> > Function hash_long() accepts unsigned long, while currently only one byte
> > is passed from ima_hash_key(), which calculates a key for ima_htable.
> >
> > Given that hashing the digest does not give clear benefits compared to
> > using the digest itself, remove hash_long() and return the modulus
> > calculated on the beginning of the digest with the number of slots. Also
> > reduce the depth of the hash table by doubling the number of slots.
> >
> > Cc: stable@xxxxxxxxxxxxxxx
> > Fixes: 3323eec921ef ("integrity: IMA as an integrity service provider")
> > Co-developed-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > Signed-off-by: Krzysztof Struczynski <krzysztof.struczynski@xxxxxxxxxx>
> > ---
> >  security/integrity/ima/ima.h | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
> > index 467dfdbea25c..6ee458cf124a 100644
> > --- a/security/integrity/ima/ima.h
> > +++ b/security/integrity/ima/ima.h
> > @@ -36,7 +36,7 @@ enum tpm_pcrs { TPM_PCR0 = 0, TPM_PCR8 = 8 };
> >  #define IMA_DIGEST_SIZE		SHA1_DIGEST_SIZE
> >  #define IMA_EVENT_NAME_LEN_MAX	255
> >
> > -#define IMA_HASH_BITS 9
> > +#define IMA_HASH_BITS 10
> >  #define IMA_MEASURE_HTABLE_SIZE (1 << IMA_HASH_BITS)
> >
> >  #define IMA_TEMPLATE_FIELD_ID_MAX_LEN	16
> > @@ -179,9 +179,9 @@ struct ima_h_table {
> >  };
> >  extern struct ima_h_table ima_htable;
> >
> > -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?

Thanks

Roberto

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> 	David
> 
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