PCI/MSI: Don't apply affinity if there aren't enough vectors left

Bart reported a problem wіth an out of bounds access in the low-level IRQ
affinity code, which we root caused to the qla2xxx driver assigning all its
MSI-X vectors to the pre and post vectors, and not having any left for the
actually spread IRQs.

Fix this issue by not asking for affinity assignment when there are no
vectors to assign left.

Fixes: 402723ad ("PCI/MSI: Provide pci_alloc_irq_vectors_affinity()")
Link: https://lkml.kernel.org/r/1485359225.3093.3.camel@sandisk.comReported-by: Bart Van Assche <bart.vanassche@sandisk.com>
Tested-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

drivers/pci/msi.c

@@ -1206,6 +1206,16 @@ int pci_alloc_irq_vectors_affinity(struct pci_dev *dev, unsigned int min_vecs,
 	if (flags & PCI_IRQ_AFFINITY) {
 		if (!affd)
 			affd = &msi_default_affd;
+
+		if (affd->pre_vectors + affd->post_vectors > min_vecs)
+			return -EINVAL;
+
+		/*
+		 * If there aren't any vectors left after applying the pre/post
+		 * vectors don't bother with assigning affinity.
+		 */
+		if (affd->pre_vectors + affd->post_vectors == min_vecs)
+			affd = NULL;
 	} else {
 		if (WARN_ON(affd))
 			affd = NULL;