[PATCH] x86: don't assume all fb devices are PCI devices

[Vitaly Kuznetsov]

When booting Hyper-V Generation 2 guests KASAN reports the following
out-of-bounds access:

BUG: KASAN: slab-out-of-bounds in fb_is_primary_device+0x58/0x70 at addr
  ffff880079cf0eb0
Read of size 8 by task swapper/0/1
...
 [<ffffffff81581308>] dump_stack+0x63/0x8b
 [<ffffffff812e1f99>] print_trailer+0xf9/0x150
 [<ffffffff812e7344>] object_err+0x34/0x40
 [<ffffffff812e9630>] kasan_report_error+0x230/0x550
 [<ffffffff812e9ee8>] kasan_report+0x58/0x60
 [<ffffffff812e4500>] ? ___slab_alloc+0x80/0x490
 [<ffffffff81878a28>] ? fb_is_primary_device+0x58/0x70
 [<ffffffff812e87cd>] __asan_load8+0x5d/0x70
 [<ffffffff81878a28>] fb_is_primary_device+0x58/0x70
 [<ffffffff8162357a>] register_framebuffer+0xda/0x5b0
 [<ffffffff816234a0>] ? remove_conflicting_framebuffers+0x50/0x50
...

The issue is caused by the to_pci_dev() call with no check that the given
info->device is in fact a pci device and some fb devices (Hyper-V FB, EFI
FB,...) are not. fb_is_primary_device() is not on any performance critical
path, replace to_pci_dev() with raw scan.

Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
---
 arch/x86/video/fbdev.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/arch/x86/video/fbdev.c b/arch/x86/video/fbdev.c
index d5644bb..f309d6c 100644
--- a/arch/x86/video/fbdev.c
+++ b/arch/x86/video/fbdev.c
@@ -14,12 +14,18 @@
 int fb_is_primary_device(struct fb_info *info)
 {
 	struct device *device = info->device;
-	struct pci_dev *pci_dev = NULL;
+	struct pci_dev *dev = NULL, *pci_dev = NULL;
 	struct pci_dev *default_device = vga_default_device();
 	struct resource *res = NULL;
 
-	if (device)
-		pci_dev = to_pci_dev(device);
+	/*
+	 * We're not sure info->device is a pci device, do full scan instead
+	 * of to_pci_dev().
+	 */
+	for_each_pci_dev(dev) {
+		if (&dev->dev == device)
+			pci_dev = dev;
+	}
 
 	if (!pci_dev)
 		return 0;
-- 
2.5.0