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

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Date:	Thu, 31 Mar 2016 15:31:42 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Alexander Popov <alpopov@...ecurity.com>
Cc:	Peter Jones <pjones@...hat.com>,
	Matt Fleming <matt@...eblueprint.co.uk>,
	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>, x86@...nel.org,
	linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org
Subject: Re: [PATCH RFC 1/1] x86: fix bad memory access in
 fb_is_primary_device()


* Alexander Popov <alpopov@...ecurity.com> wrote:

> 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.

No need to worry, it's all upstream already.

Thanks,

	Ingo