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

[David Laight]

From: Roberto Sassu
> Sent: 28 April 2020 08:30
> From: Krzysztof Struczynski <krzysztof.struczynski@xxxxxxxxxx>
> 
> 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 first two bytes of the digest with the number of slots.
> Also reduce the depth of the hash table by doubling the number of slots.
> 
> Changelog
> 
> v2: directly access the first two bytes of the digest to avoid memory
>     access issues on big endian systems (suggested by David Laight)
> 
> 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>

Acked-by: David.Laight@xxxxxxxxxx

> ---
>  security/integrity/ima/ima.h | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
> index 467dfdbea25c..02796473238b 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 +179,10 @@ 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);
> +	/* there is no point in taking a hash of part of a digest */
> +	return (digest[0] | digest[1] << 8) % IMA_MEASURE_HTABLE_SIZE;
>  }
> 
>  #define __ima_hooks(hook)		\
> --
> 2.17.1

-
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