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SubjectRe: [PATCH v9 5/6] IMA: Add support to limit measuring keys
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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

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