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

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Date:	Mon, 14 Mar 2016 10:52:28 +0100
From:	Vitaly Kuznetsov <vkuznets@...hat.com>
To:	Bjorn Helgaas <helgaas@...nel.org>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Cathy Avery <cavery@...hat.com>,
	"K. Y. Srinivasan" <kys@...rosoft.com>
Subject: Re: [PATCH] x86: don't assume all fb devices are PCI devices

Bjorn Helgaas <helgaas@...nel.org> writes:

> 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@...hat.com>
>> ---
>>  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;

Sure, will do v2.

>
>   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.

Yes, the original code is for pci devices only and I didn't plan to
change that, my intention is to prevent out-of-bounds access.

>
> Bjorn

-- 
  Vitaly