Re: [FFmpeg-devel] [PATCH v2] avfilter/vf_scale: Cleanup some checks

[Kacper Michajlow <kasper93@gmail.com>]

On Tue, 9 Jul 2024 at 15:17, Anton Khirnov <anton@khirnov.net> wrote:
>
> Quoting Michael Niedermayer (2024-07-09 13:37:11)
> > Fixes: CID1513722 Operands don't affect result
> >
> > Sponsored-by: Sovereign Tech Fund
> > Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> > ---
> >  libavfilter/vf_scale.c | 6 ++----
> >  1 file changed, 2 insertions(+), 4 deletions(-)
> >
> > diff --git a/libavfilter/vf_scale.c b/libavfilter/vf_scale.c
> > index bf09196e10d..18e9393d6c1 100644
> > --- a/libavfilter/vf_scale.c
> > +++ b/libavfilter/vf_scale.c
> > @@ -645,10 +645,8 @@ static int config_props(AVFilterLink *outlink)
> >      if (ret < 0)
> >          goto fail;
> >
> > -    if (outlink->w > INT_MAX ||
> > -        outlink->h > INT_MAX ||
> > -        (outlink->h * inlink->w) > INT_MAX ||
> > -        (outlink->w * inlink->h) > INT_MAX)
> > +    if ((outlink->h * (int64_t)inlink->w) > INT32_MAX ||
> > +        (outlink->w * (int64_t)inlink->h) > INT32_MAX)
>
> This does not seems cleaner to me.
>
> Also, this check overall seems fishy. Why is it here at all and not e.g.
> in ff_scale_adjust_dimensions()? Why does it not call
> av_image_check_size()? Why does it only print a warning and not do
> anything else?

I agree with Anton here. The checks in vf_scale are iffy at best.

For starters, this is `outlink->w > INT_MAX` dead check. As the `w` is
int already. And there is already an UB in `scale_eval_dimensions()`
which converts double value to int without any checks.

I think something like this would make sense to reject big numbers,
and ofcourse ff_scale_adjust_dimensions() should be more clever about
overflows too. There is also an overflow in swscale, but that's
another story.

diff --git a/libavfilter/vf_scale.c b/libavfilter/vf_scale.c
index a1a322ed9e..9483db7564 100644
--- a/libavfilter/vf_scale.c
+++ b/libavfilter/vf_scale.c
@@ -537,7 +537,7 @@ static int scale_eval_dimensions(AVFilterContext *ctx)
const AVPixFmtDescriptor *desc = av_pix_fmt_desc_get(inlink->format);
const AVPixFmtDescriptor *out_desc = av_pix_fmt_desc_get(outlink->format);
char *expr;
- int eval_w, eval_h;
+ double eval_w, eval_h;
int ret;
double res;
const AVPixFmtDescriptor *main_desc;
@@ -588,23 +588,22 @@ static int scale_eval_dimensions(AVFilterContext *ctx)
}

res = av_expr_eval(scale->w_pexpr, scale->var_values, NULL);
- eval_w = scale->var_values[VAR_OUT_W] = scale->var_values[VAR_OW] =
(int) res == 0 ? inlink->w : (int) res;
+ eval_w = scale->var_values[VAR_OUT_W] = scale->var_values[VAR_OW] =
res == 0 ? inlink->w : res;

res = av_expr_eval(scale->h_pexpr, scale->var_values, NULL);
- if (isnan(res)) {
- expr = scale->h_expr;
+ eval_h = scale->var_values[VAR_OUT_H] = scale->var_values[VAR_OH] =
res == 0 ? inlink->h : res;
+
+ if (isnan(eval_w) || eval_w < INT_MIN || eval_w > INT_MAX) {
+ expr = scale->w_expr;
ret = AVERROR(EINVAL);
goto fail;
}
- eval_h = scale->var_values[VAR_OUT_H] = scale->var_values[VAR_OH] =
(int) res == 0 ? inlink->h : (int) res;

- res = av_expr_eval(scale->w_pexpr, scale->var_values, NULL);
- if (isnan(res)) {
- expr = scale->w_expr;
+ if (isnan(eval_h) || eval_h < INT_MIN || eval_h > INT_MAX) {
+ expr = scale->h_expr;
ret = AVERROR(EINVAL);
goto fail;
}
- eval_w = scale->var_values[VAR_OUT_W] = scale->var_values[VAR_OW] =
(int) res == 0 ? inlink->w : (int) res;

scale->w = eval_w;
scale->h = eval_h;
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