[FFmpeg-devel] [PATCH] nutdec: reject negative sm_size

[Michael Niedermayer]

On Sat, Dec 19, 2015 at 12:19:44PM +0100, Andreas Cadhalpun wrote:
> On 19.12.2015 01:51, Michael Niedermayer wrote:
> > On Fri, Dec 18, 2015 at 05:22:31PM +0100, Andreas Cadhalpun wrote:
> >> If it is negative, it makes size larger than the size of the packet
> >> buffer, causing invalid writes in avio_read.
> >>
> >> Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun at googlemail.com>
> >> ---
> >>  libavformat/nutdec.c | 5 +++++
> >>  1 file changed, 5 insertions(+)
> >>
> >> diff --git a/libavformat/nutdec.c b/libavformat/nutdec.c
> >> index 286d1ee..47ae7a7 100644
> >> --- a/libavformat/nutdec.c
> >> +++ b/libavformat/nutdec.c
> >> @@ -1146,6 +1146,11 @@ static int decode_frame(NUTContext *nut, AVPacket *pkt, int frame_code)
> >>              goto fail;
> >>          }
> >>          sm_size = avio_tell(bc) - pkt->pos;
> >> +        if (sm_size < 0) {
> > 
> > did sm_size overflow and should be 64bit ?
> 
> No.
> 
> > did the byte position (avio_tell) move backward ?
> 
> Yes.
> 
> > (this should not happen)
> 
> In that case, the check needs to be moved to read_sm_data.
> Patch doing that is attached.
> 
> Best regards,
> Andreas
> 

>  nutdec.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 2f0ac251ee05a8a36ffbaec5a9d5346ac0ef4240  0001-nutdec-reject-negative-value_len-in-read_sm_data.patch
> From bdca159087d426f5f989656a08464fec967b4bc3 Mon Sep 17 00:00:00 2001
> From: Andreas Cadhalpun <Andreas.Cadhalpun at googlemail.com>
> Date: Sat, 19 Dec 2015 12:02:56 +0100
> Subject: [PATCH] nutdec: reject negative value_len in read_sm_data
> 
> If it is negative, it can cause the byte position to move backwards in
> avio_skip, which in turn makes sm_size negative and thus size larger
> than the size of the packet buffer, causing invalid writes in avio_read.
> 
> Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun at googlemail.com>
> ---
>  libavformat/nutdec.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/libavformat/nutdec.c b/libavformat/nutdec.c
> index 286d1ee..b33b3e2 100644
> --- a/libavformat/nutdec.c
> +++ b/libavformat/nutdec.c
> @@ -934,7 +934,7 @@ static int read_sm_data(AVFormatContext *s, AVIOContext *bc, AVPacket *pkt, int
>                  return ret;
>              }
>              value_len = ffio_read_varlen(bc);
> -            if (avio_tell(bc) + value_len >= maxpos)
> +            if (value_len < 0 || avio_tell(bc) + value_len >= maxpos)
>                  return AVERROR_INVALIDDATA;

ok, also while at it please fix avio_tell(bc) + value_len, which i
belive can overflow
you could alternatively change value_len to uint64_t which might
simplify the check


[...]


-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

It is what and why we do it that matters, not just one of them.
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