PCI: Fix infinite loop with ROM image of size 0

If the image size would ever read as 0, pci_get_rom_size() could keep
processing the same image over and over again.  Exit the loop if we ever
read a length of zero.

This fixes a soft lockup on boot when the radeon driver calls
pci_get_rom_size() on an AMD Radeon R7 250X PCIe discrete graphics card.

[bhelgaas: changelog, reference]
Link: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1386973Reported-by: Federico <federicotg@gmail.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
CC: stable@vger.kernel.org

drivers/pci/rom.c

@@ -71,6 +71,7 @@ size_t pci_get_rom_size(struct pci_dev *pdev, void __iomem *rom, size_t size)
 {
 	void __iomem *image;
 	int last_image;
+	unsigned length;
 
 	image = rom;
 	do {
@@ -93,9 +94,9 @@ size_t pci_get_rom_size(struct pci_dev *pdev, void __iomem *rom, size_t size)
 		if (readb(pds + 3) != 'R')
 			break;
 		last_image = readb(pds + 21) & 0x80;
-		/* this length is reliable */
-		image += readw(pds + 16) * 512;
-	} while (!last_image);
+		length = readw(pds + 16);
+		image += length * 512;
+	} while (length && !last_image);
 
 	/* never return a size larger than the PCI resource window */
 	/* there are known ROMs that get the size wrong */