Re: [PATCH for-8.2] ui/pixman-minimal.h: fix empty allocation

[Marc-André Lureau]

Hi Manos

On Tue, Nov 21, 2023 at 1:38 PM Manos Pitsidianakis
<manos.pitsidianakis@linaro.org> wrote:
>
> In the minimal pixman API stub that is used when the real pixman
> dependency is missing a NULL dereference happens when
> virtio-gpu-rutabaga allocates a pixman image with bits = NULL and
> rowstride_bytes = zero. A buffer of rowstride_bytes * height is
> allocated which is NULL. However, in that scenario pixman calculates a
> new stride value based on given width, height and format size.
>
> This commit adds a helper function that performs the same logic as
> pixman.
>

Thanks a lot for investigating this and providing a solution!
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>

> Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
> ---
>  include/ui/pixman-minimal.h | 48 +++++++++++++++++++++++++++++++++++--
>  1 file changed, 46 insertions(+), 2 deletions(-)
>
> diff --git a/include/ui/pixman-minimal.h b/include/ui/pixman-minimal.h
> index efcf570c9e..6dd7de1c7e 100644
> --- a/include/ui/pixman-minimal.h
> +++ b/include/ui/pixman-minimal.h
> @@ -113,6 +113,45 @@ typedef struct pixman_color {
>      uint16_t    alpha;
>  } pixman_color_t;
>
> +static inline uint32_t *create_bits(pixman_format_code_t format,
> +                                    int width,
> +                                    int height,
> +                                    int *rowstride_bytes)
> +{
> +    int stride = 0;
> +    size_t buf_size = 0;
> +    int bpp = PIXMAN_FORMAT_BPP(format);
> +
> +    /*
> +     * Calculate the following while checking for overflow truncation:
> +     * stride = ((width * bpp + 0x1f) >> 5) * sizeof(uint32_t);
> +     */
> +
> +    if (unlikely(__builtin_mul_overflow(width, bpp, &stride))) {
> +        return NULL;
> +    }
> +
> +    if (unlikely(__builtin_add_overflow(stride, 0x1f, &stride))) {
> +        return NULL;
> +    }
> +
> +    stride >>= 5;
> +
> +    stride *= sizeof(uint32_t);
> +
> +    if (unlikely(__builtin_mul_overflow((size_t) height,
> +                                        (size_t) stride,
> +                                        &buf_size))) {
> +        return NULL;
> +    }
> +
> +    if (rowstride_bytes) {
> +        *rowstride_bytes = stride;
> +    }
> +
> +    return g_malloc0(buf_size);
> +}
> +
>  static inline pixman_image_t *pixman_image_create_bits(pixman_format_code_t 
> format,
>                                                         int width,
>                                                         int height,
> @@ -123,13 +162,18 @@ static inline pixman_image_t 
> *pixman_image_create_bits(pixman_format_code_t form
>
>      i->width = width;
>      i->height = height;
> -    i->stride = rowstride_bytes ?: width * 
> DIV_ROUND_UP(PIXMAN_FORMAT_BPP(format), 8);
>      i->format = format;
>      if (bits) {
>          i->data = bits;
>      } else {
> -        i->free_me = i->data = g_malloc0(rowstride_bytes * height);
> +        i->free_me = i->data =
> +            create_bits(format, width, height, &rowstride_bytes);
> +        if (width && height) {
> +            assert(i->data);
> +        }
>      }
> +    i->stride = rowstride_bytes ? rowstride_bytes :
> +                            width * DIV_ROUND_UP(PIXMAN_FORMAT_BPP(format), 
> 8);
>      i->ref_count = 1;
>
>      return i;
>
> base-commit: af9264da80073435fd78944bc5a46e695897d7e5
> --
> 2.39.2
>