On 06/06/2012 03:03 AM, Yan, Zheng wrote:
> From: "Yan, Zheng" <[email protected]>
>
> The bug can cause NULL pointer dereference in write_partial_msg_pages
>
> Signed-off-by: Zheng Yan <[email protected]>
> ---
> net/ceph/messenger.c | 1 +
> 1 files changed, 1 insertions(+), 0 deletions(-)
>
> diff --git a/net/ceph/messenger.c b/net/ceph/messenger.c
> index 1a80907..785b953 100644
> --- a/net/ceph/messenger.c
> +++ b/net/ceph/messenger.c
> @@ -598,6 +598,7 @@ static void prepare_write_message(struct ceph_connection
> *con)
> le32_to_cpu(con->out_msg->footer.front_crc),
> le32_to_cpu(con->out_msg->footer.middle_crc));
>
> + m->bio_iter = NULL;
> /* is there a data payload? */
> if (le32_to_cpu(m->hdr.data_len) > 0) {
> /* initialize page iterator */
OK, now that I've looked at this a little bit I think I have a small
preference for doing this assignment earlier in that function, where
there is already special handling that is dependent on whether the
message being processed is being re-sent or not:
/*
* only assign outgoing seq # if we haven't sent this message
* yet. if it is requeued, resend with it's original seq.
*/
if (m->needs_out_seq) {
m->hdr.seq = cpu_to_le64(++con->out_seq);
m->needs_out_seq = false;
}
So I'd suggest it look like:
/*
* only assign outgoing seq # if we haven't sent this message
* yet. if it is requeued, resend with it's original seq.
*/
if (m->needs_out_seq) {
m->hdr.seq = cpu_to_le64(++con->out_seq);
m->needs_out_seq = false;
}
#ifdef CONFIG_BLOCK
else {
/* Need to reinitialize the iterator on resends */
m->bio_iter = NULL;
}
#endif
Please let me know if you concur with my proposed modification or
whether you feel strongly we should go with your original fix.
In any case, I think your fix looks like the right thing to do.
Reviewed-by: Alex Elder <[email protected]>
Now let me know if you see anything wrong in the following explanation
of the problem.
The first time a message is sent, it will have been allocated via
ceph_msg_new(), which will initialize both the bio and bio_iter
fields to NULL (and bio_seg to 0).
rbd is an osd client. It sets up its request queue with
request_queue->request_fn = rbd_rq_fn()
So here's the path that leads to a bio in a message:
rbd_rq_fn()
->rbd_req_{read,write}()
->rbd_do_op()
->rbd_do_request()
->ceph_osdc_alloc_request()
And rbd_osdc_alloc_request() assigns the passed-in bio to req->r_bio
and grabs a reference to it.
Later, ceph_osdc_start_request() will copy the osd request's r_bio
pointer value into the request message that will be sent to the osd
server. It does not update either of the other two bio-related fields
in that message--in particular, bio_iter is still NULL.
Then, in write_partial_msg_pages(), this code uses the null bio_iter
as an indicator that we're just starting with processing the message
that is currently referred to in con->out_msg, and initializes the
iteration fields.
#ifdef CONFIG_BLOCK
if (msg->bio && !msg->bio_iter)
init_bio_iter(msg->bio, &msg->bio_iter, &msg->bio_seg);
#endif
So...
In order for this to work properly, msg->bio_iter needs to be
a null pointer when we're first processing a message in
write_partial_msg_pages(). And that's fine the first time through,
but if a connection failure occurs in the middle of processing
a bio request, and ceph_fault() gets called, all sent messages get
re-queued to be re-sent, and at that point, msg->bio_iter is not
guaranteed to be a null pointer.
(I worked through describing all this because in doing so I can
convince myself I understand the problem enough to know this is
the right fix.)
-Alex
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