Re: [PATCH 2/6] integrity: IMA as an integrity service provider

[david safford <safford@watson.ibm.com>]

On Mon, 2009-02-02 at 17:02 -0600, Serge E. Hallyn wrote:
> Quoting Mimi Zohar (zohar@linux.vnet.ibm.com):
> > +
> > +/* Add template entry to the measurement list and hash table,
> > + * and extend the pcr.
> > + */
> > +int ima_add_template_entry(struct ima_template_entry *entry, int violation,
> > +			   const char *op, struct inode *inode)
> > +{
> > +	u8 digest[IMA_DIGEST_SIZE];
> > +	const char *audit_cause = "hash_added";
> > +	int audit_info = 1;
> > +	int result = 0;
> > +
> > +	mutex_lock(&ima_extend_list_mutex);
> > +	if (!violation) {
> > +		memcpy(digest, entry->digest, sizeof digest);
> > +		if (ima_lookup_digest_entry(digest)) {
> > +			audit_cause = "hash_exists";
> > +			goto out;
> 
> Ok so not that I'm saying this would be easy, but an attacker
> compromising say ftpd doesn't need to come up with a compromised
> ftpd where sha1sum(evilftpd)==sha1sum(origftpd) - he just needs to
> come up with one wher sha1sum(evilftpd)==sha1sum(X) where X is
> any pristine program already loaded.  Right?
> 
> Is checking that strcmp(entry->file_name, newfilename)==0 warranted
> here, or am I being silly?

The birthday paradox, which is essential for any practical attack,
requires that the attacker can freely choose both x and y in 
his search for collisions sha1sum(x) == sha1sum(y). Here, the
attacker cannot freely choose x, but can only choose x from a list
of existing files. Even if there are a million valid files, that only
reduces the attack complexity by 2^20, which is still infeasible.

dave