[PATCH 4.2.y-ckt 239/305] xen/pciback: Return error on XEN_PCI_OP_enable_msi when device has MSI or MSI-X enabled

4.2.8-ckt2 -stable review patch.  If anyone has any objections, please let me know.

---8<------------------------------------------------------------

From: Konrad Rzeszutek Wilk <redacted>

commit 56441f3c8e5bd45aab10dd9f8c505dd4bec03b0d upstream.

The guest sequence of:

 a) XEN_PCI_OP_enable_msi
 b) XEN_PCI_OP_enable_msi
 c) XEN_PCI_OP_disable_msi

results in hitting an BUG_ON condition in the msi.c code.

The MSI code uses an dev->msi_list to which it adds MSI entries.
Under the above conditions an BUG_ON() can be hit. The device
passed in the guest MUST have MSI capability.

The a) adds the entry to the dev->msi_list and sets msi_enabled.
The b) adds a second entry but adding in to SysFS fails (duplicate entry)
and deletes all of the entries from msi_list and returns (with msi_enabled
is still set).  c) pci_disable_msi passes the msi_enabled checks and hits:

BUG_ON(list_empty(dev_to_msi_list(&dev->dev)));

and blows up.

The patch adds a simple check in the XEN_PCI_OP_enable_msi to guard
against that. The check for msix_enabled is not stricly neccessary.

This is part of XSA-157.

Reviewed-by: David Vrabel <redacted>
Reviewed-by: Jan Beulich <redacted>
Signed-off-by: Konrad Rzeszutek Wilk <redacted>
Signed-off-by: Kamal Mostafa <redacted>
---
 drivers/xen/xen-pciback/pciback_ops.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/xen/xen-pciback/pciback_ops.c b/drivers/xen/xen-pciback/pciback_ops.c
index a0e0e3e..8bfb87c 100644
--- a/drivers/xen/xen-pciback/pciback_ops.c
+++ b/drivers/xen/xen-pciback/pciback_ops.c

@@ -144,7 +144,12 @@ int xen_pcibk_enable_msi(struct xen_pcibk_device *pdev,
 	if (unlikely(verbose_request))
 		printk(KERN_DEBUG DRV_NAME ": %s: enable MSI\n", pci_name(dev));
 
-	status = pci_enable_msi(dev);
+	if (dev->msi_enabled)
+		status = -EALREADY;
+	else if (dev->msix_enabled)
+		status = -ENXIO;
+	else
+		status = pci_enable_msi(dev);
 
 	if (status) {
 		pr_warn_ratelimited("%s: error enabling MSI for guest %u: err %d\n",

-- 
1.9.1