| Message ID | 20220714224410.51147-1-sj@kernel.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v2] xen-blkback: fix persistent grants negotiation | expand |
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;
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;
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(-)