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

From: Bjorn Helgaas
Date: Sat Mar 12 2016 - 15:59:13 EST


On Fri, Mar 11, 2016 at 03:39:18PM +0100, Vitaly Kuznetsov wrote:
> 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;

I think dev_is_pci() is a better solution:

if (!device || !dev_is_pci(device))
return 0;

pci_dev = to_pci_dev(device);
if (vga_default_device()) {
if (vga_default_device() == pci_dev)
return 1;
return 0;
}

res = &pci_dev->resource[PCI_ROM_RESOURCE];
if (res->flags & IORESOURCE_ROM_SHADOW)
return 1;

return 0;

This says a non-PCI device cannot be the primary device. I don't know
framebuffers, so I don't know if that's what you really want, but I
guess it's what the original code says.

Bjorn