Re: [PATCH v2] x86: don't assume all fb devices are PCI devices

From: Ingo Molnar <mingo@kernel.org>
To: Bjorn Helgaas <helgaas@kernel.org>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	Cathy Avery <cavery@redhat.com>,
	"K. Y. Srinivasan" <kys@microsoft.com>
Subject: Re: [PATCH v2] x86: don't assume all fb devices are PCI devices
Date: Mon, 14 Mar 2016 17:33:45 +0100	[thread overview]
Message-ID: <20160314163345.GA19628@gmail.com> (raw)
In-Reply-To: <20160314152807.GA13471@localhost>

* Bjorn Helgaas <helgaas@kernel.org> wrote:

> On Mon, Mar 14, 2016 at 03:42:57PM +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.
> > 
> > Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> 
> Acked-by: Bjorn Helgaas <bhelgaas@google.com>
> 
> > ---
> > Changes since v1: use dev_is_pci() instead of full scan [Bjorn Helgaas].
> > ---
> >  arch/x86/video/fbdev.c | 7 +++----
> >  1 file changed, 3 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/x86/video/fbdev.c b/arch/x86/video/fbdev.c
> > index d5644bb..9f2fe63 100644
> > --- a/arch/x86/video/fbdev.c
> > +++ b/arch/x86/video/fbdev.c
> > @@ -18,12 +18,11 @@ int fb_is_primary_device(struct fb_info *info)
> >  	struct pci_dev *default_device = vga_default_device();
> >  	struct resource *res = NULL;
> 
> The initializations of "pci_dev" and "res" to NULL are now
> unnecessary.
> 
> >  
> > -	if (device)
> > -		pci_dev = to_pci_dev(device);
> > -
> > -	if (!pci_dev)
> > +	if (!device || !dev_is_pci(device))
> >  		return 0;
> >  
> > +	pci_dev = to_pci_dev(device);
> > +
> >  	if (default_device) {
> >  		if (pci_dev == default_device)
> >  			return 1;
> 
> The test for non-NULL "res" below is superfluous because we know
> "pci_dev" is non-NULL:
> 
>   res = &pci_dev->resource[PCI_ROM_RESOURCE];
>   if (res && res->flags & IORESOURCE_ROM_SHADOW)
> 
> I think your patch is functionally correct, so it's up to you and the
> x86 guys whether you want to do these additional cleanups.

I'd like to see those cleanups as well, no point in delaying them, right?

btw., I'd suggest writing this:

>   res = &pci_dev->resource[PCI_ROM_RESOURCE];

as:

>   res = pci_dev->resource + PCI_ROM_RESOURCE;

Thanks,

	Ingo