xen-blkback: fix persistent grants negotiation

commit bd4a11959f36a0f202dd5328f65a811674de3d7e upstream.

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 3d14b04b1cc9 ("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: 3d14b04b1cc9 ("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>
Reviewed-by: Maximilian Heyne <mheyne@amazon.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20220715225108.193398-2-sj@kernel.org
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index 33eba3df4dd9af6f545856326a46378178c0db27..82696d47046aff8d8fb190e98eb06d9f19b2b011 100644 (file)
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -156,6 +156,11 @@ 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;
@@ -180,6 +185,8 @@ 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;
 }
 
@@ -471,12 +478,6 @@ 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)
@@ -522,8 +523,6 @@ static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle,
        if (q && blk_queue_secure_erase(q))
                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;