linux-kernel - Re: [PATCH v9 5/6] IMA: Add support to limit measuring keys

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Date:   Mon, 02 Dec 2019 13:18:34 -0500
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
        linux-integrity@...r.kernel.org
Cc:     eric.snowberg@...cle.com, dhowells@...hat.com,
        matthewgarrett@...gle.com, sashal@...nel.org,
        jamorris@...ux.microsoft.com, linux-kernel@...r.kernel.org,
        keyrings@...r.kernel.org
Subject: Re: [PATCH v9 5/6] IMA: Add support to limit measuring keys

On Wed, 2019-11-27 at 16:44 -0800, Lakshmi Ramasubramanian wrote:
> On 11/27/19 10:52 AM, Mimi Zohar wrote:
> 
> Hi Mimi,
> 
> >> +static bool ima_match_keyring(struct ima_rule_entry *rule,
> >> +			      const char *keyring)
> >> +{
> >> +	/*
> >> +	 * "keyrings=" is specified in the policy in the format below:
> >> +	 *   keyrings=.builtin_trusted_keys|.ima|.evm
> >> +	 *
> >> +	 * Each keyring name in the option is separated by a '|' and
> >> +	 * the last keyring name is null terminated.
> >> +	 *
> >> +	 * The given keyring is considered matched only if
> >> +	 * the whole keyring name matched a keyring name specified
> >> +	 * in the "keyrings=" option.
> >> +	 */
> >> +	p = strstr(rule->keyrings, keyring);
> >> +	if (p) {
> >> +		/*
> >> +		 * Found a substring match. Check if the character
> >> +		 * at the end of the keyring name is | (keyring name
> >> +		 * separator) or is the terminating null character.
> >> +		 * If yes, we have a whole string match.
> >> +		 */
> >> +		p += strlen(keyring);
> >> +		if (*p == '|' || *p == '\0')
> >> +			return true;

This code checks that the keyring name isn't suffixed, but not
prefixed.

> >> +	}
> >> +
> > 
> > Using "while strsep()" would simplify this code, removing the need for
> > such a long comment.
> > 
> > Mimi
> 
> strsep() modifies the source string (replaces the delimiter with '\0' 
> and also updates the source string pointer). I am not sure it can be 
> used for our scenario. Please correct me if I am wrong.
> 
> Initial IMA policy:
> -------------------
> measure func=KEY_CHECK 
> keyrings=.ima|.evm|.builtin_trusted_keys|.blacklist template=ima-buf
> 
> Policy after adding a key to .ima keyring:
> ------------------------------------------
> measure func=KEY_CHECK keyrings=.evm|.builtin_trusted_keys|.blacklist 
> template=ima-buf
> 
> Policy after adding a key to a keyring that is not listed in the policy:
> ------------------------------------------------------------------------
> measure func=KEY_CHECK keyrings= template=ima-buf
> 
> ********************************************************************************
> 
> Please see the description from the man page for strsep():
> 
> http://man7.org/linux/man-pages/man3/strsep.3.html
> 
> char *strsep(char **stringp, const char *delim);
> 
> This function finds the first token in the string *stringp, that is 
> delimited by one of the bytes in the string delim.  This token is 
> terminated by overwriting the delimiter with a null byte ('\0'), and 
> *stringp is updated to point past the token.

Yes, you would have to make a copy of the string before using
strsep().  You could always use kstrdup(), remembering to free it, or
allocate the memory just once, and then just use memcpy.

Mimi