linux-kernel - Re: [PATCH RFC 1/1] x86: fix bad memory access in fb_is_primary

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Date:	Fri, 25 Mar 2016 14:29:21 +0300
From:	Alexander Popov <alpopov@...ecurity.com>
To:	Peter Jones <pjones@...hat.com>,
	Matt Fleming <matt@...eblueprint.co.uk>,
	Ingo Molnar <mingo@...nel.org>, Arnd Bergmann <arnd@...db.de>,
	Tomi Valkeinen <tomi.valkeinen@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>
CC:	<x86@...nel.org>, <linux-kernel@...r.kernel.org>,
	<linux-efi@...r.kernel.org>,
	Alexander Popov <alpopov@...ecurity.com>
Subject: Re: [PATCH RFC 1/1] x86: fix bad memory access in
 fb_is_primary_device()

On 09.03.2016 15:46, Alexander Popov wrote:
> On 16.02.2016 18:18, Peter Jones wrote:
>> On Tue, Feb 16, 2016 at 01:49:18PM +0000, Matt Fleming wrote:
>>> [ Including Peter, the efifb maintainer. Original email is here,
>>>
>>>     http://marc.info/?l=linux-kernel&m=145552936131335&w=2
>>>
>>>   I've snipped some of the quoted text ]
>>>
>>> On Tue, 16 Feb, at 08:55:22AM, Ingo Molnar wrote:
>>>>
>>>> (I've Cc:-ed the EFI-FB and FB gents. Mail quoted below.)
>>>>
>>>> * Alexander Popov <alpopov@...ecurity.com> wrote:
>>>>
>>>>> Currently the code in fb_is_primary_device() contains to_pci_dev() macro
>>>>> which is applied to dev from struct fb_info. In some cases this causes
>>>>> bad memory access when fb_is_primary_device() handles fb_info of efifb.
>>>>> The reason is that fb dev of efifb is embedded into struct platform_device
>>>>> but not into struct pci_dev.
>>>>>
>>>>> We can fix this by checking fb dev bus name in fb_is_primary_device().
>>>>>
>>>>> It seems that this bug reveals some bigger problem with to_pci_dev(),
>>>>> to_platform_device() and others, which just do container_of() and
>>>>> don't check whether struct device is a part of the appropriate structure.
>>>>> Should we do something more about it?
>>>>>
>>>>> KASan report:
>>>
>>> [...]
>>>
>>>>>
>>>>> Signed-off-by: Alexander Popov <alpopov@...ecurity.com>
>>>>> ---
>>>>>  arch/x86/video/fbdev.c | 9 +++++----
>>>>>  1 file changed, 5 insertions(+), 4 deletions(-)
>>>>>
>>>>> diff --git a/arch/x86/video/fbdev.c b/arch/x86/video/fbdev.c
>>>>> index d5644bb..4999f78 100644
>>>>> --- a/arch/x86/video/fbdev.c
>>>>> +++ b/arch/x86/video/fbdev.c
>>>>> @@ -18,11 +18,12 @@ int fb_is_primary_device(struct fb_info *info)
>>>>>  	struct pci_dev *default_device = vga_default_device();
>>>>>  	struct resource *res = NULL;
>>>>>  
>>>>> -	if (device)
>>>>> -		pci_dev = to_pci_dev(device);
>>>>> -
>>>>> -	if (!pci_dev)
>>>>> +	if (!device || !device->bus ||
>>>>> +		    !device->bus->name || strcmp(device->bus->name, "pci")) {
>>>>>  		return 0;
>>>>> +	}
>>>>> +
>>>>> +	pci_dev = to_pci_dev(device);
>>>>>  
>>>>>  	if (default_device) {
>>>>>  		if (pci_dev == default_device)
>>>>> -- 
>>>>> 1.9.1
>>>>>
>>>
>>> I wonder if this issue could explain some of the efifb issues we've
>>> seen reported on bugzilla.kernel.org in the past where switching from
>>> efifb to some other framebuffer device caused hangs during boot. I'm
>>> struggling to find the relevant bugzilla entries now, though.
>>
>> It's possible it could, but I don't have them handy either. 

[...]

>> So it's most likely right for efifb to be embedded in a platform_device
>> instead of a pci_dev.  Which leads back to Alexander's question - if it
>> isn't in a pci_dev, that means fb_is_primary_device() needs to not
>> assume it is.  So the patch appears correct, but so is the question -
>> should to_pci_dev() be checking this and returning NULL here?
> 
> The discussion has suspended. May I activate it again?
> 
> So there are two ways to fix the bad memory access in fb_is_primary_device().
> 
> The first one is proposed in my patch. Checking the bus name string doesn't
> look good but I didn't manage to come up with anything better.
> 
> The second way is changing to_pci_dev() similarly. It may return NULL or
> call BUG() when struct device is a part of an inappropriate structure.
> 
> Which way is better? Do we need to do anything with other similar macros?

Excuse me, there is no reply for a long time. Did I touch any taboo topic?
Hope to fix this bug. Thanks.

Best regards,
Alexander