Re: [PATCH 3/8] xen/blkfront: don't take local copy of a request from the ring page

[Juergen Gross]

On 17.05.21 16:01, Jan Beulich wrote:

On 13.05.2021 12:02, Juergen Gross wrote:

In order to avoid a malicious backend being able to influence the local
copy of a request build the request locally first and then copy it to
the ring page instead of doing it the other way round as today.

Signed-off-by: Juergen Gross <jgr...@suse.com>

Reviewed-by: Jan Beulich <jbeul...@suse.com>
with one remark/question:

@@ -703,6 +704,7 @@ static int blkif_queue_rw_req(struct request *req,

struct blkfront_ring_info *ri

  {
        struct blkfront_info *info = rinfo->dev_info;
        struct blkif_request *ring_req, *extra_ring_req = NULL;
+       struct blkif_request *final_ring_req, *final_extra_ring_req;

Without setting final_extra_ring_req to NULL just like is done for
extra_ring_req, ...

@@ -840,10 +845,10 @@ static int blkif_queue_rw_req(struct request *req, struct 
blkfront_ring_info *ri
        if (setup.segments)
                kunmap_atomic(setup.segments);

-       rinfo->shadow[id].req = *ring_req;
+       /* Copy request(s) to the ring page. */
+       *final_ring_req = *ring_req;
        if (unlikely(require_extra_req))
-               rinfo->shadow[extra_id].req = *extra_ring_req;
+               *final_extra_ring_req = *extra_ring_req;

- /* Keep a private copy so we can reissue requests when recovering. */

... are you sure all supported compilers will recognize the
conditional use and not warn about use of a possibly uninitialized
variable?

Hmm, probably better safe than sorry. Will change it.


Juergen

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