netfs: Fix the (non-)cancellation of copy when cache is temporarily disabled

JIRA: https://issues.redhat.com/browse/RHEL-75660
CVE: CVE-2024-57941

commit d0327c8
Author: David Howells <dhowells@redhat.com>
Date:   Fri Dec 13 13:50:10 2024 +0000

    netfs: Fix the (non-)cancellation of copy when cache is temporarily disabled

    When the caching for a cookie is temporarily disabled (e.g. due to a DIO
    write on that file), future copying to the cache for that file is disabled
    until all fds open on that file are closed.  However, if netfslib is using
    the deprecated PG_private_2 method (such as is currently used by ceph), and
    decides it wants to copy to the cache, netfs_advance_write() will just bail
    at the first check seeing that the cache stream is unavailable, and
    indicate that it dealt with all the content.

    This means that we have no subrequests to provide notifications to drive
    the state machine or even to pin the request and the request just gets
    discarded, leaving the folios with PG_private_2 set.

    Fix this by jumping directly to cancel the request if the cache is not
    available.  That way, we don't remove mark3 from the folio_queue list and
    netfs_pgpriv2_cancel() will clean up the folios.

    This was found by running the generic/013 xfstest against ceph with an
    active cache and the "-o fsc" option passed to ceph.  That would usually
    hang

    Fixes: ee4cdf7 ("netfs: Speed up buffered reading")
    Reported-by: Max Kellermann <max.kellermann@ionos.com>
    Closes: https://lore.kernel.org/r/CAKPOu+_4m80thNy5_fvROoxBm689YtA0dZ-=gcmkzwYSY4syqw@mail.gmail.com/
    Signed-off-by: David Howells <dhowells@redhat.com>
    Link: https://lore.kernel.org/r/20241213135013.2964079-11-dhowells@redhat.com
    cc: Jeff Layton <jlayton@kernel.org>
    cc: Ilya Dryomov <idryomov@gmail.com>
    cc: Xiubo Li <xiubli@redhat.com>
    cc: netfs@lists.linux.dev
    cc: ceph-devel@vger.kernel.org
    cc: linux-fsdevel@vger.kernel.org
    Signed-off-by: Christian Brauner <brauner@kernel.org>

Signed-off-by: CKI Backport Bot <cki-ci-bot+cki-gitlab-backport-bot@redhat.com>

Author: CKI Backport Bot

fs/netfs/read_pgpriv2.c

@@ -170,6 +170,10 @@ void netfs_pgpriv2_write_to_the_cache(struct netfs_io_request *rreq)
 
 	trace_netfs_write(wreq, netfs_write_trace_copy_to_cache);
 	netfs_stat(&netfs_n_wh_copy_to_cache);
+	if (!wreq->io_streams[1].avail) {
+		netfs_put_request(wreq, false, netfs_rreq_trace_put_return);
+		goto couldnt_start;
+	}
 
 	for (;;) {
 		error = netfs_pgpriv2_copy_folio(wreq, folio);