linux-kernel - Re: [PATCH] x86: simplefb: avoid overflow

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Message-ID: <CANq1E4QueHY41Kz5xWnMC5Y8fQzsb4a7FxyrYg2XuGLOQ-wZfw@mail.gmail.com>
Date:	Fri, 6 Sep 2013 13:24:52 +0200
From:	David Herrmann <dh.herrmann@...il.com>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	Tom Gundersen <teg@...m.no>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86: simplefb: avoid overflow

Hi

On Fri, Sep 6, 2013 at 12:59 PM, Geert Uytterhoeven
<geert@...ux-m68k.org> wrote:
> On Fri, Sep 6, 2013 at 11:55 AM, David Herrmann <dh.herrmann@...il.com> wrote:
>> On Fri, Sep 6, 2013 at 11:32 AM, Tom Gundersen <teg@...m.no> wrote:
>>> lfb_size can easily be say 4M, which would make the bitshit overflow and
>>> the test fail.
>>>
>>> Signed-off-by: Tom Gundersen <teg@...m.no>
>>> Cc: David Herrmann <dh.herrmann@...il.com>
>>> Cc: H. Peter Anvin <hpa@...or.com>
>>> ---
>>>  arch/x86/kernel/sysfb_simplefb.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/arch/x86/kernel/sysfb_simplefb.c b/arch/x86/kernel/sysfb_simplefb.c
>>> index 22513e9..fff44a5 100644
>>> --- a/arch/x86/kernel/sysfb_simplefb.c
>>> +++ b/arch/x86/kernel/sysfb_simplefb.c
>>> @@ -72,7 +72,7 @@ __init int create_simplefb(const struct screen_info *si,
>>>          * the part that is occupied by the framebuffer */
>>>         len = mode->height * mode->stride;
>>>         len = PAGE_ALIGN(len);
>>> -       if (len > si->lfb_size << 16) {
>>> +       if (len > ((unsigned long) si->lfb_size) << 16) {
>
> On 32-bit, "unsigned long" is the same size as __u32, so this doesn't
> make any difference.

lfb_size cannot be 4M on 32bit machines. Well, if it is, the firmware
passed bogus information as you cannot have a 262G-region on 32bit.
But on 64bit it can pass 4M just fine.
So we don't care for the 32bit case here, only 64bit. The (__u64) cast
would be more obvious. Don't know.. It's just a sanity check, anyways,
so I'm fine with it.

>> Nice catch. vesafb uses "lfb_size * 65535" which causes an implicit
>> cast. I thought <<16 looks nicer but that doesn't do any implicit
>> cast..
>
> "lfb_size * 65535" is the same. "lfb_size" is __u32, "65535" is int.
> So there's no implicit cast. Or am I missing something?

Yepp, indeed. My bad, so vesafb doesn't do any better here.

I wonder, though, which firmware passes such values. It means the
lfb_size area is reserved >4G. We do have huge VMEM these days..
Tom, on what hardware did you hit that?

Thanks
David
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