vfio/pci: Disable auto-enable of exclusive INTx IRQ

Currently for devices requiring masking at the irqchip for INTx, ie.
devices without DisINTx support, the IRQ is enabled in request_irq()
and subsequently disabled as necessary to align with the masked status
flag.  This presents a window where the interrupt could fire between
these events, resulting in the IRQ incrementing the disable depth twice.
This would be unrecoverable for a user since the masked flag prevents
nested enables through vfio.

Instead, invert the logic using IRQF_NO_AUTOEN such that exclusive INTx
is never auto-enabled, then unmask as required.

Cc:  <stable@vger.kernel.org>
Fixes: 89e1f7d ("vfio: Add PCI device driver")
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Link: https://lore.kernel.org/r/20240308230557.805580-2-alex.williamson@redhat.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

Author: Alex Williamson

drivers/vfio/pci/vfio_pci_intrs.c

@@ -296,8 +296,15 @@ static int vfio_intx_set_signal(struct vfio_pci_core_device *vdev, int fd)
 
 	ctx->trigger = trigger;
 
+	/*
+	 * Devices without DisINTx support require an exclusive interrupt,
+	 * IRQ masking is performed at the IRQ chip.  The masked status is
+	 * protected by vdev->irqlock. Setup the IRQ without auto-enable and
+	 * unmask as necessary below under lock.  DisINTx is unmodified by
+	 * the IRQ configuration and may therefore use auto-enable.
+	 */
 	if (!vdev->pci_2_3)
-		irqflags = 0;
+		irqflags = IRQF_NO_AUTOEN;
 
 	ret = request_irq(pdev->irq, vfio_intx_handler,
 			  irqflags, ctx->name, vdev);
@@ -308,13 +315,9 @@ static int vfio_intx_set_signal(struct vfio_pci_core_device *vdev, int fd)
 		return ret;
 	}
 
-	/*
-	 * INTx disable will stick across the new irq setup,
-	 * disable_irq won't.
-	 */
 	spin_lock_irqsave(&vdev->irqlock, flags);
-	if (!vdev->pci_2_3 && ctx->masked)
-		disable_irq_nosync(pdev->irq);
+	if (!vdev->pci_2_3 && !ctx->masked)
+		enable_irq(pdev->irq);
 	spin_unlock_irqrestore(&vdev->irqlock, flags);
 
 	return 0;