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

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Date:   Mon, 27 Apr 2020 11:00:06 +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@...wei.com" <silviu.vlasceanu@...wei.com>,
        "krzysztof.struczynski@...wei.com" <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 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@...r.kernel.org
> Fixes: 3323eec921ef ("integrity: IMA as an integrity service provider")
> Co-developed-by: Roberto Sassu <roberto.sassu@...wei.com>
> Signed-off-by: Roberto Sassu <roberto.sassu@...wei.com>
> Signed-off-by: Krzysztof Struczynski <krzysztof.struczynski@...wei.com>
> ---
>  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.

	David

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