[PATCH v2] xen-blkback: fix persistent grants negotiation

[SeongJae Park <sj@kernel.org>]

Persistent grants feature can be used only when both backend and the
frontend supports the feature.  The feature was always supported by
'blkback', but commit aac8a70db24b ("xen-blkback: add a parameter for
disabling of persistent grants") has introduced a parameter for
disabling it runtime.

To avoid the parameter be updated while being used by 'blkback', the
commit caches the parameter into 'vbd->feature_gnt_persistent' in
'xen_vbd_create()', and then check if the guest also supports the
feature and finally updates the field in 'connect_ring()'.

However, 'connect_ring()' could be called before 'xen_vbd_create()', so
later execution of 'xen_vbd_create()' can wrongly overwrite 'true' to
'vbd->feature_gnt_persistent'.  As a result, 'blkback' could try to use
'persistent grants' feature even if the guest doesn't support the
feature.

This commit fixes the issue by moving the parameter value caching to
'xen_blkif_alloc()', which allocates the 'blkif'.  Because the struct
embeds 'vbd' object, which will be used by 'connect_ring()' later, this
should be called before 'connect_ring()' and therefore this should be
the right and safe place to do the caching.

Fixes: aac8a70db24b ("xen-blkback: add a parameter for disabling of persistent grants")
Cc: <stable@vger.kernel.org> # 5.10.x
Signed-off-by: Maximilian Heyne <mheyne@amazon.de>
Signed-off-by: SeongJae Park <sj@kernel.org>
---

Changes from v1[1]
- Avoid the behavioral change[2]
- Rebase on latest xen/tip/linux-next
- Re-work by SeongJae Park <sj@kernel.org>
- Cc stable@

[1] https://lore.kernel.org/xen-devel/20220106091013.126076-1-mheyne@amazon.de/
[2] https://lore.kernel.org/xen-devel/20220121102309.27802-1-sj@kernel.org/

 drivers/block/xen-blkback/xenbus.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index 97de13b14175..16c6785d260c 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -157,6 +157,11 @@ static int xen_blkif_alloc_rings(struct xen_blkif *blkif)
 	return 0;
 }
 
+/* Enable the persistent grants feature. */
+static bool feature_persistent = true;
+module_param(feature_persistent, bool, 0644);
+MODULE_PARM_DESC(feature_persistent, "Enables the persistent grants feature");
+
 static struct xen_blkif *xen_blkif_alloc(domid_t domid)
 {
 	struct xen_blkif *blkif;
@@ -181,6 +186,8 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
 	__module_get(THIS_MODULE);
 	INIT_WORK(&blkif->free_work, xen_blkif_deferred_free);
 
+	blkif->vbd.feature_gnt_persistent = feature_persistent;
+
 	return blkif;
 }
 
@@ -472,12 +479,6 @@ static void xen_vbd_free(struct xen_vbd *vbd)
 	vbd->bdev = NULL;
 }
 
-/* Enable the persistent grants feature. */
-static bool feature_persistent = true;
-module_param(feature_persistent, bool, 0644);
-MODULE_PARM_DESC(feature_persistent,
-		"Enables the persistent grants feature");
-
 static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle,
 			  unsigned major, unsigned minor, int readonly,
 			  int cdrom)
@@ -520,8 +521,6 @@ static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle,
 	if (bdev_max_secure_erase_sectors(bdev))
 		vbd->discard_secure = true;
 
-	vbd->feature_gnt_persistent = feature_persistent;
-
 	pr_debug("Successful creation of handle=%04x (dom=%u)\n",
 		handle, blkif->domid);
 	return 0;
-- 
2.25.1