CVE-2021-47391

In the Linux kernel, the following vulnerability has been resolved:

RDMA/cma: Ensure rdma_addr_cancel() happens before issuing more requests

The FSM can run in a circle allowing rdma_resolve_ip() to be called twice on the same id_priv. While this cannot happen without going through the work, it violates the invariant that the same address resolution background request cannot be active twice.

   CPU 1                                  CPU 2

rdma_resolve_addr(): RDMA_CM_IDLE -> RDMA_CM_ADDR_QUERY rdma_resolve_ip(addr_handler) #1

		 process_one_req(): for #1
                      addr_handler():
                        RDMA_CM_ADDR_QUERY -> RDMA_CM_ADDR_BOUND
                        mutex_unlock(&id_priv->handler_mutex);
                        [.. handler still running ..]

rdma_resolve_addr(): RDMA_CM_ADDR_BOUND -> RDMA_CM_ADDR_QUERY rdma_resolve_ip(addr_handler) !! two requests are now on the req_list

rdma_destroy_id(): destroy_id_handler_unlock(): _destroy_id(): cma_cancel_operation(): rdma_addr_cancel()

                      // process_one_req() self removes it
	          spin_lock_bh(&lock);
                       cancel_delayed_work(&req->work);
                   if (!list_empty(&req->list)) == true

  ! rdma_addr_cancel() returns after process_on_req #1 is done

kfree(id_priv)

		 process_one_req(): for #2
                      addr_handler():
                    mutex_lock(&id_priv->handler_mutex);
                        !! Use after free on id_priv

rdma_addr_cancel() expects there to be one req on the list and only cancels the first one. The self-removal behavior of the work only happens after the handler has returned. This yields a situations where the req_list can have two reqs for the same handle but rdma_addr_cancel() only cancels the first one.

The second req remains active beyond rdma_destroy_id() and will use-after-free id_priv once it inevitably triggers.

Fix this by remembering if the id_priv has called rdma_resolve_ip() and always cancel before calling it again. This ensures the req_list never gets more than one item in it and doesnt cost anything in the normal flow that never uses this strange error path.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/03d884671572af8bcfbc9e63944c1021efce7589
• https://git.kernel.org/stable/c/305d568b72f17f674155a2a8275f865f207b3808
• https://git.kernel.org/stable/c/9a085fa9b7d644a234465091e038c1911e1a4f2a
• https://git.kernel.org/stable/c/03d884671572af8bcfbc9e63944c1021efce7589
• https://git.kernel.org/stable/c/305d568b72f17f674155a2a8275f865f207b3808
• https://git.kernel.org/stable/c/9a085fa9b7d644a234465091e038c1911e1a4f2a