Re: [FFmpeg-devel] [PATCH] libavformat/usmdec: add support for HCA stream decryption

[Pavel Roslyy <roslypav-at-gmail.com@ffmpeg.org>]

On Thu, Jun 26, 2025 at 2:08 PM Michael Niedermayer
<michael@niedermayer.cc> wrote:
>
> Hi Pavel
>
> On Thu, Jun 26, 2025 at 12:04:17AM -0700, Pavel Roslyy wrote:
> > On Wed, Jun 25, 2025 at 3:40 PM Michael Niedermayer
> > <michael@niedermayer.cc> wrote:
> > >
> > > [...]
> > >
> > > bug found, not applying yet
> > >
> > > ret = ff_alloc_extradata(par, pkt_size + key_buf);
> > >
> > > pkt_size + key_buf can overflow i think
> >
> > If I'm understanding right, I don't think it can.
> > pkt_size = chunk_size - (ret - chunk_start) - padding_size;
> >
> > (ret - chunk_start) should be at least 24 at this point, and I don't think
> > padding_size will be negative so pkt_size is at most UINT32_MAX - 24.
>
> chunk_size is arbitrary 32bit thus pkt_size is arbitrary 32bit
>
>
> >
> > key_buf adds at most 10, which is not enough to overflow.
>
> arbitrary uint32_t + 10 can overflow. Its a defined overflow
> but the following allocation is then bad

I think what happens is arbitrary uint32_t - 24 + 10, which cannot overflow
but it looks like I wasn't convincing.  I assume you want an overflow check?

if (key_buf > UINT32_MAX - pkt_size)
    return AVERROR_INVALIDDATA;

Would this work or do you have a better suggestion?

Regards,
Pavel Roslyy
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