[FFmpeg-devel] ABI break in 4.3

[Carl Eugen Hoyos]

Am So., 5. Juli 2020 um 01:38 Uhr schrieb Andreas Rheinhardt
<andreas.rheinhardt at gmail.com>:

> This crash is due to Chromium using av_max_alloc in an undocumented way;
> more exactly, the Chromium FFmpeg fork changes the allocation functions
> so that if max_alloc_size is zero, the limit is completely disabled; and
> of course it also sets the limit to zero. Up until 4.2, this worked with
> a normal FFmpeg, because FFmpeg didn't max_alloc_size as is, but instead
> max_alloc_size - 32. But since 731c77589841c02e014aa7f8285dcfc8b20f2ee5
> this is no longer so.

I think it is not immediately obvious that this is a (severe!) issue in
Chromium which basically disabled a security feature of FFmpeg
that was intentionally set to a very conservative (read: not soo
secure) value but was completely disabled by somebody who
misunderstood the feature (and failed to ask, I mention this
because this person's understanding would have implied that we
have no clue in C programming whatsoever).

At least one of the "downstream" fixes I saw in the last weeks simply
repeat this failure by again removing the security feature instead of
removing the wrong call from Chromium.

I am not sure if it really is our responsibility to explain to downstream
that valid multimedia files theoretically can allocate arbitrary amounts
of memory but that a responsible caller has to limit this amount for
nearly every theoretical use case, the more so for browser decoding.

Carl Eugen