On 15.07.22 01:44, SeongJae Park wrote:

Hello all.

Adding Andrii Chepurnyi to CC who have played with the use-case which required reconnect recently and faced some issues with feature_persistent handling.

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;