[FFmpeg-devel] [PATCH 2/2] avformat/mpc8: fix hang with fuzzed file

[Reimar Döffinger]

On Tue, Feb 03, 2015 at 09:54:51PM +0100, wm4 wrote:
> On Tue, 3 Feb 2015 21:47:57 +0100
> Reimar Döffinger <Reimar.Doeffinger at gmx.de> wrote:
> 
> > On Tue, Feb 03, 2015 at 07:04:12PM +0100, wm4 wrote:
> > > This can lead to an endless loop by seeking back a few bytes after each
> > > attempted chunk read. Assuming negative sizes are always invalid, this
> > > is easy to fix. Other code in this demuxer treats negative sizes as
> > > invalid as well.
> > > 
> > > Fixes ticket #4262.
> > > ---
> > >  libavformat/mpc8.c | 4 ++++
> > >  1 file changed, 4 insertions(+)
> > > 
> > > diff --git a/libavformat/mpc8.c b/libavformat/mpc8.c
> > > index d6ca338..6524c7e 100644
> > > --- a/libavformat/mpc8.c
> > > +++ b/libavformat/mpc8.c
> > > @@ -223,6 +223,10 @@ static int mpc8_read_header(AVFormatContext *s)
> > >      while(!avio_feof(pb)){
> > >          pos = avio_tell(pb);
> > >          mpc8_get_chunk_header(pb, &tag, &size);
> > > +        if (size < 0) {
> > 
> > Isn't the only way for this to become negative for a too
> > large uint64_t to be assigned to a int64_t?
> > I.e. undefined behaviour.
> > In that case this isn't quite the right way in the strictest sense,
> > though it is likely to work "normally".
> 
> This is what mpc8_get_chunk_header() does:
> 
>     pos = avio_tell(pb);
>     *tag = avio_rl16(pb);
>     *size = ffio_read_varlen(pb);
>     *size -= avio_tell(pb) - pos;
> 
> ffio_read_varlen() returns uint64_t, so I don't think it's supposed to
> read negative values. But the last line could make it negative anyway
> if the chunk size field read here is inconsistent.

The problem is in this line:
*size = ffio_read_varlen(pb);
*size is int64_t.
If ffio_read_varlen returns a value > 0x7f..ffull then this
assignment is undefined, which a fairly risky thing to allow
for a security-relevant check.
That is ignoring the fact that I am not exactly happy that
ffio_read_varlen will happily read until EOF of a multi-GB file,
which doesn't seem like reasonable behaviour at least in this
use-case.