[PATCH 6/7] drm/crtc: add sanity checks to create_dumb()

[David Herrmann]

Hi

On Tue, Jan 21, 2014 at 10:49 AM, Daniel Vetter <daniel at ffwll.ch> wrote:
> On Mon, Jan 20, 2014 at 08:26:28PM +0100, David Herrmann wrote:
>> Lets make sure some basic expressions are always true:
>>   bpp != NULL
>>   width != NULL
>>   height != NULL
>>   stride = bpp * width < 2^32
>>   size = stride * height < 2^32
>>   PAGE_ALIGN(size) < 2^32
>>
>> At least the udl driver doesn't check for multiplication-overflows, so
>> lets just make sure it will never happen. These checks allow drivers to do
>> any 32bit math without having to test for mult-overflows themselves.
>>
>> The two divisions might hurt performance a bit, but dumb_create() is only
>> used for scanout-buffers, so that should be fine. We could use 64bit math
>> to avoid the divisions, but that may be slow on 32bit machines.. Or maybe
>> there should just be a "safe_mult32()" helper, which currently doesn't
>> exist (I think?).
>>
>> Signed-off-by: David Herrmann <dh.herrmann at gmail.com>
>> ---
>>  drivers/gpu/drm/drm_crtc.c | 15 +++++++++++++++
>>  1 file changed, 15 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
>> index 266a01d..ff647fa 100644
>> --- a/drivers/gpu/drm/drm_crtc.c
>> +++ b/drivers/gpu/drm/drm_crtc.c
>> @@ -3738,9 +3738,24 @@ int drm_mode_create_dumb_ioctl(struct drm_device *dev,
>>                              void *data, struct drm_file *file_priv)
>>  {
>>       struct drm_mode_create_dumb *args = data;
>> +     u32 Bpp, stride, size;
>
> s/Bpp/bpp/

It's actually "Bytes per pixel", not "Bits per pixel". We generally
use "bpp" as "bits per pixel" so I'd like to avoid the confusion. But
yeah, upper-case names are unusual so I can also use bytes_pp or sth
similar?

>>
>>       if (!dev->driver->dumb_create)
>>               return -ENOSYS;
>> +     if (!args->width || !args->height || !args->bpp)
>> +             return -EINVAL;
>> +
>> +     /* overflow checks for 32bit size calculations */
>> +     Bpp = (args->bpp + 7) / 8;
>
> Again DIV_ROUND_UP

yepp, fixed.

>
>> +     if (Bpp > 0xffffffffU / args->width)
>> +             return -EINVAL;
>> +     stride = Bpp * args->width;
>> +     if (args->height > 0xffffffffU / stride)
>> +             return -EINVAL;
>> +     size = args->height * stride;
>> +     if (PAGE_ALIGN(size) < size)
>
> Maybe check for 0 here and add a small comment that this checks
> wrap-around.

Hm, the comment above those if()s already says "overflow checks", and
it should be obvious that all three are overflow checks, so I don't
think we need another comment (especially because I didn't add any
empty lines between them).

I will change it to "if (!PAGE_ALIGN(size))". The "x + off < x" is an
addition-overflow-check so I think it doesn't apply here.

Thanks
David

>
>> +             return -EINVAL;
>> +
>>       return dev->driver->dumb_create(file_priv, dev, args);
>>  }
>>
>> --
>> 1.8.5.3
>>
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> +41 (0) 79 365 57 48 - http://blog.ffwll.ch