The 17 agencies are those listed in Schedule 3 of C-51.
The relevant sections are sections 5 and 6 of C-51:
Disclosure of information
5. (1) Subject to any provision of any other Act of Parliament, or of any regulation made under such an Act, that prohibits or restricts the disclosure of information, a Government of Canada institution may, on its own initiative or on request, disclose information to the head of a recipient Government of Canada institution whose title is listed in Schedule 3, or their delegate, if the information is relevant to the recipient institution’s jurisdiction or responsibilities under an Act of Parliament or another lawful authority in respect of activities that undermine the security of Canada, including in respect of their detection, identification, analysis, prevention, investigation or disruption.
(2) Information received under subsection (1) may be further disclosed under that subsection.
6. For greater certainty, nothing in this Act prevents a head, or their delegate, who receives information under subsection 5(1) from, in accordance with the law, using that information, or further disclosing it to any person, for any purpose.
There's a few other sections updating other acts (Excise Act, Income Tax Act, etc.) to allow information sharing, too.
The 17 agencies are:
The three with oversight are CSIS (by the Security Intelligence Review Committee (SIRC)), CSE (the Office of the CSE Commissioner (OCSEC)), and the RCMP (the Commission for Public Complaints against the RMCP (CPC)).