[FFmpeg-devel] [PATCH 1/2] avcodec: add mvdv video decoder

[Tomas Härdin]

ons 2019-11-27 klockan 20:00 +0100 skrev Paul B Mahol:
> On 11/27/19, James Almer <jamrial at gmail.com> wrote:
> > On 11/27/2019 3:42 PM, Paul B Mahol wrote:
> > > On 11/27/19, Tomas Härdin <tjoppen at acc.umu.se> wrote:
> > > > tis 2019-11-26 klockan 20:29 +0100 skrev Paul B Mahol:
> > > > > On 11/26/19, James Almer <jamrial at gmail.com> wrote:
> > > > > > On 11/26/2019 6:47 AM, Paul B Mahol wrote:
> > > > > > > On 11/25/19, Tomas Härdin <tjoppen at acc.umu.se> wrote:
> > > > > > > > mån 2019-11-25 klockan 22:09 +0100 skrev Paul B Mahol:
> > > > > > > > > Signed-off-by: Paul B Mahol <onemda at gmail.com>
> > > > > > > > > +static int decode_mvdv(MidiVidContext *s, AVCodecContext *avctx,
> > > > > > > > > AVFrame
> > > > > > > > > *frame)
> > > > > > > > > +{
> > > > > > > > > +    GetByteContext *gb = &s->gb;
> > > > > > > > > +    GetBitContext mask;
> > > > > > > > > +    GetByteContext idx9;
> > > > > > > > > +    uint16_t nb_vectors, intra_flag;
> > > > > > > > > +    const uint8_t *vec;
> > > > > > > > > +    const uint8_t *mask_start;
> > > > > > > > > +    uint8_t *skip;
> > > > > > > > > +    int mask_size;
> > > > > > > > > +    int idx9bits = 0;
> > > > > > > > > +    int idx9val = 0;
> > > > > > > > > +    int num_blocks;
> > > > > > > > > +
> > > > > > > > > +    nb_vectors = bytestream2_get_le16(gb);
> > > > > > > > > +    intra_flag = bytestream2_get_le16(gb);
> > > > > > > > > +    if (intra_flag) {
> > > > > > > > > +        num_blocks = (avctx->width / 2) * (avctx->height / 2);
> > > > > > > > 
> > > > > > > > Will UB if width*height/4 > INT_MAX
> > > > > > > > 
> > > > > > > > > +    } else {
> > > > > > > > > +        int skip_linesize;
> > > > > > > > > +
> > > > > > > > > +        num_blocks = bytestream2_get_le32(gb);
> > > > > > > > 
> > > > > > > > Might want to use uint32_t so this doesn't lead to weirdness on
> > > > > > > > 32-bit
> > > > > > > > 
> > > > > > > > > +        skip_linesize = avctx->width >> 1;
> > > > > > > > > +        mask_start = gb->buffer_start + bytestream2_tell(gb);
> > > > > > > > > +        mask_size = (avctx->width >> 5) * (avctx->height >> 2);
> > > > > > > > 
> > > > > > > > This can also UB
> > > > > > > > 
> > > > > > > > /Tomas
> > > > > > > > 
> > > > > > > 
> > > > > > > Nothing of this can actually happen.
> > > > 
> > > > This assumes max_pixels will never be increased above INT_MAX. "64k"
> > > > video is most definitely within the range of possibility in the coming
> > > > years, if it isn't already. Film archival and DPX come to mind.
> > > > 
> > > > > > It can and i'm fairly sure it will happen as soon as the fuzzer starts
> > > > > > testing this decoder using big dimensions.
> > > > > 
> > > > > I'm not that guy doing such work. Please stop bikesheding those
> > > > > patches for once.
> > > > 
> > > > This reads like an admission of pushing insecure code via "not my
> > > > problem"
> > > > 
> > > > > > You don't need asserts here, you just need to check the calculations
> > > > > > will not overflow. Do something like "if ((int64_t)avctx->width *
> > > > > > avctx->height / 4 > INT_MAX) return AVERROR_INVALIDDATA" and call it a
> > > > > > day.
> > > > > > Also, maybe num_blocks should be unsigned, seeing you set it using
> > > > > > bytestream2_get_le32() for P-frames.
> > > > > 
> > > > > No decoder does this.
> > > > 
> > > > zmbv does
> > > > 
> > > > All this is really about the lack of any way to reason about
> > > > assumptions like "dimensions can't be larger than X*Y" at compile time,
> > > > which is a thing I've been pointing out on this list for a while now.
> > > > 
> > > 
> > > Nobody tells you not to fix it yourself.
> > 
> > Just add a the suggested overflow checks, Christ. It's a one line
> > addition each. What do you gain arguing like this?
> 
> Than next day he or you will come with another great idea.

Great ideas like pushing inevitable 0days?

/Tomas