Re: [PATCH] x86: simplefb: avoid overflow

[Tom Gundersen]

On Fri, Sep 6, 2013 at 1:24 PM, David Herrmann <dh.herrmann@xxxxxxxxx> wrote:
> Hi
>
> On Fri, Sep 6, 2013 at 12:59 PM, Geert Uytterhoeven
> <geert@xxxxxxxxxxxxxx> wrote:
>> On Fri, Sep 6, 2013 at 11:55 AM, David Herrmann <dh.herrmann@xxxxxxxxx> wrote:
>>> On Fri, Sep 6, 2013 at 11:32 AM, Tom Gundersen <teg@xxxxxxx> wrote:
>>>> lfb_size can easily be say 4M, which would make the bitshit overflow and
>>>> the test fail.
>>>>
>>>> Signed-off-by: Tom Gundersen <teg@xxxxxxx>
>>>> Cc: David Herrmann <dh.herrmann@xxxxxxxxx>
>>>> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
>>>> ---
>>>>  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.

I used unsigned long simply to match the type of len...

>>> 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?

As always when weird things happen:

DMI: Apple Inc. MacBookAir5,1/Mac-66F35F19FE2A0D05, BIOS
MBA51.88Z.00EF.B01.1207271122 07/27/2012

Cheers,

Tom
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