[PATCH v2] xen/pcifront: check that an AER callback exists before calling it

[Yehyeong Lee <yhlee@xxxxxxxxxxxxxxxxxx>]

pcifront_common_process() handles an AER request from the backend by
dispatching on aer_op.cmd to the bound driver's PCI error handler.  It
only checks that err_handler and err_handler->error_detected are present,
then for the mmio_enabled, slot_reset and resume commands it calls the
corresponding callback unconditionally.  Those three callbacks are
optional -- the PCI core NULL-checks each of them individually before
use -- and many drivers (for example igb, igc, ice and ixgbevf) install
error_detected without all of them.

aer_op.cmd comes from the shared ring, so a malicious or buggy backend
can send XEN_PCI_OP_aer_mmio (or _slotreset/_resume) for a device whose
driver leaves that callback NULL and make the frontend call through a
NULL pointer, crashing the guest.

Check each callback before calling it.  When mmio_enabled or slot_reset is
absent, return PCI_ERS_RESULT_RECOVERED so recovery proceeds, as the PCI
core does for a missing callback; returning PCI_ERS_RESULT_NONE would
instead make xen-pciback tear the guest domain down.

Fixes: 956a9202cd12 ("xen-pcifront: Xen PCI frontend driver.")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Yehyeong Lee <yhlee@xxxxxxxxxxxxxxxxxx>
---
v2: return PCI_ERS_RESULT_RECOVERED for an absent mmio_enabled/slot_reset
    instead of PCI_ERS_RESULT_NONE, which xen-pciback treats as fatal and
    uses to tear the domain down.  (Bjorn acked v1; the ack is dropped here
    because v2 changes the value returned for an absent callback.)
v1: 
https://lore.kernel.org/all/20261005133019.284053-1-yhlee@xxxxxxxxxxxxxxxxxx/

There is an in-flight fix for a refcount leak in this same function
("xen/pcifront: Fix PCI device reference leak in AER handling"); this
change is orthogonal and applies in either order.

 drivers/pci/xen-pcifront.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/drivers/pci/xen-pcifront.c b/drivers/pci/xen-pcifront.c
index cffc32d660327..5c6f7796d204a 100644
--- a/drivers/pci/xen-pcifront.c
+++ b/drivers/pci/xen-pcifront.c
@@ -599,11 +599,16 @@ static pci_ers_result_t pcifront_common_process(int cmd,
                case XEN_PCI_OP_aer_detected:
                        return pdrv->err_handler->error_detected(pcidev, state);
                case XEN_PCI_OP_aer_mmio:
-                       return pdrv->err_handler->mmio_enabled(pcidev);
+                       if (pdrv->err_handler->mmio_enabled)
+                               return pdrv->err_handler->mmio_enabled(pcidev);
+                       return PCI_ERS_RESULT_RECOVERED;
                case XEN_PCI_OP_aer_slotreset:
-                       return pdrv->err_handler->slot_reset(pcidev);
+                       if (pdrv->err_handler->slot_reset)
+                               return pdrv->err_handler->slot_reset(pcidev);
+                       return PCI_ERS_RESULT_RECOVERED;
                case XEN_PCI_OP_aer_resume:
-                       pdrv->err_handler->resume(pcidev);
+                       if (pdrv->err_handler->resume)
+                               pdrv->err_handler->resume(pcidev);
                        return PCI_ERS_RESULT_NONE;
                default:
                        dev_err(&pdev->xdev->dev,
-- 
2.43.0