[PATCH 2/2] xen/virtio: Fix potential deadlock when accessing xen_grant_dma_devices

[Oleksandr Tyshchenko <olekstysh@xxxxxxxxx>]

From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>

As find_xen_grant_dma_data() is called from both interrupt and process
contexts, the access to xen_grant_dma_devices XArray must be protected
by xa_lock_irqsave to avoid deadlock scenario.
As XArray API doesn't provide xa_store_irqsave helper, call lockless
__xa_store directly and guard it externally.

Also move the storage of the XArray's entry to a separate helper.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
Fixes: d6aca3504c7d ("xen/grant-dma-ops: Add option to restrict memory access 
under Xen")
---
 drivers/xen/grant-dma-ops.c | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/drivers/xen/grant-dma-ops.c b/drivers/xen/grant-dma-ops.c
index 1998d0e8ce82..c66f56d24013 100644
--- a/drivers/xen/grant-dma-ops.c
+++ b/drivers/xen/grant-dma-ops.c
@@ -25,7 +25,7 @@ struct xen_grant_dma_data {
        bool broken;
 };
 
-static DEFINE_XARRAY(xen_grant_dma_devices);
+static DEFINE_XARRAY_FLAGS(xen_grant_dma_devices, XA_FLAGS_LOCK_IRQ);
 
 #define XEN_GRANT_DMA_ADDR_OFF (1ULL << 63)
 
@@ -42,14 +42,29 @@ static inline grant_ref_t dma_to_grant(dma_addr_t dma)
 static struct xen_grant_dma_data *find_xen_grant_dma_data(struct device *dev)
 {
        struct xen_grant_dma_data *data;
+       unsigned long flags;
 
-       xa_lock(&xen_grant_dma_devices);
+       xa_lock_irqsave(&xen_grant_dma_devices, flags);
        data = xa_load(&xen_grant_dma_devices, (unsigned long)dev);
-       xa_unlock(&xen_grant_dma_devices);
+       xa_unlock_irqrestore(&xen_grant_dma_devices, flags);
 
        return data;
 }
 
+static int store_xen_grant_dma_data(struct device *dev,
+                                   struct xen_grant_dma_data *data)
+{
+       unsigned long flags;
+       int ret;
+
+       xa_lock_irqsave(&xen_grant_dma_devices, flags);
+       ret = xa_err(__xa_store(&xen_grant_dma_devices, (unsigned long)dev, 
data,
+                       GFP_ATOMIC));
+       xa_unlock_irqrestore(&xen_grant_dma_devices, flags);
+
+       return ret;
+}
+
 /*
  * DMA ops for Xen frontends (e.g. virtio).
  *
@@ -338,8 +353,7 @@ void xen_grant_setup_dma_ops(struct device *dev)
         */
        data->backend_domid = iommu_spec.args[0];
 
-       if (xa_err(xa_store(&xen_grant_dma_devices, (unsigned long)dev, data,
-                       GFP_KERNEL))) {
+       if (store_xen_grant_dma_data(dev, data)) {
                dev_err(dev, "Cannot store Xen grant DMA data\n");
                goto err;
        }
-- 
2.25.1