[PATCH v2] pvcalls-front: fix potential null dereference

[Wen Yang]

 static checker warning:
    drivers/xen/pvcalls-front.c:373 alloc_active_ring()
    error: we previously assumed 'map->active.ring' could be null
           (see line 357)

drivers/xen/pvcalls-front.c
    351 static int alloc_active_ring(struct sock_mapping *map)
    352 {
    353     void *bytes;
    354
    355     map->active.ring = (struct pvcalls_data_intf *)
    356         get_zeroed_page(GFP_KERNEL);
    357     if (!map->active.ring)
                    ^^^^^^^^^^^^^^^^^
Check

    358         goto out;
    359
    360     map->active.ring->ring_order = PVCALLS_RING_ORDER;
    361     bytes = (void *)__get_free_pages(GFP_KERNEL | __GFP_ZERO,
    362                     PVCALLS_RING_ORDER);
    363     if (!bytes)
    364         goto out;
    365
    366     map->active.data.in = bytes;
    367     map->active.data.out = bytes +
    368         XEN_FLEX_RING_SIZE(PVCALLS_RING_ORDER);
    369
    370     return 0;
    371
    372 out:
--> 373     free_active_ring(map);
                                 ^^^
Add null check on map->active.ring before dereferencing it to avoid
any NULL pointer dereferences.

Fixes: 9f51c05dc41a ("pvcalls-front: Avoid get_free_pages(GFP_KERNEL)
under spinlock")
Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Suggested-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Signed-off-by: Wen Yang <wen.yang99@xxxxxxxxxx>
CC: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
CC: Juergen Gross <jgross@xxxxxxxx>
CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
CC: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
CC: xen-devel@xxxxxxxxxxxxxxxxxxxx
CC: linux-kernel@xxxxxxxxxxxxxxx
---
v2->v1:
- Add NULL check on map->active.ring and return
  immediately if it is.

 drivers/xen/pvcalls-front.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/xen/pvcalls-front.c b/drivers/xen/pvcalls-front.c
index 307861f..8a249c9 100644
--- a/drivers/xen/pvcalls-front.c
+++ b/drivers/xen/pvcalls-front.c
@@ -343,6 +343,9 @@ int pvcalls_front_socket(struct socket *sock)
 
 static void free_active_ring(struct sock_mapping *map)
 {
+	if (!map->active.ring)
+		return;
+
 	free_pages((unsigned long)map->active.data.in,
 			map->active.ring->ring_order);
 	free_page((unsigned long)map->active.ring);
-- 
2.9.5