[ovs-dev] [PATCH] ofpbuf: Fix use-after-free in bundle parse.

[Joe Stringer]

On 4 March 2016 at 18:00, William Tu <u9012063 at gmail.com> wrote:
> Address pointed by bundle could be obsolete/free'd when
> realloc, called from ofpbuf_put_zero(), returns new address.
> Reported by Valgrind 367: ovs-ofctl parse-flows (NXM)
>
> Invalid write of size 4
>     bundle_parse__ (bundle.c:200)
>     bundle_parse_load (bundle.c:272)
>     parse_bundle_load (ofp-actions.c:1324)
>     ofpacts_parse__ (ofp-actions.c:7484)
>     ofpacts_parse (ofp-actions.c:7540)
>     ofpacts_parse_copy (ofp-actions.c:7558)
>     parse_ofp_str__ (ofp-parse.c:491)
>     parse_ofp_str (ofp-parse.c:544)
>     parse_ofp_flow_mod_str (ofp-parse.c:870)
>
> Address 0x7a4e96c is 12 bytes inside a block of size 64 free'd
>     free (vg_replace_malloc.c:530)
>     ofpbuf_resize__ (ofpbuf.c:246) (purposely add to force using new buf)
>     ofpbuf_put_zeros (ofpbuf.c:375)
>     bundle_parse__ (bundle.c:181)
>     bundle_parse_load (bundle.c:272)
>     parse_bundle_load (ofp-actions.c:1324)
>     ofpacts_parse__ (ofp-actions.c:7484)
>     ofpacts_parse (ofp-actions.c:7540)
>     ofpacts_parse_copy (ofp-actions.c:7558)
>
> Signed-off-by: William Tu <u9012063 at gmail.com>
> ---
>  lib/bundle.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/lib/bundle.c b/lib/bundle.c
> index 871a724..1451e92 100644
> --- a/lib/bundle.c
> +++ b/lib/bundle.c
> @@ -180,6 +180,7 @@ bundle_parse__(const char *s, char **save_ptr,
>      }
>      ofpact_finish(ofpacts, &bundle->ofpact);
>
> +    bundle = ofpacts->header;
>      bundle->basis = atoi(basis);

I don't understand how this would trigger (or how this would fix the
issue); the same line is also done directly after ofpbuf_put() inside
the loop above:
https://github.com/openvswitch/ovs/blob/34abaa3deaa430ca0b50453865d2e042a5132165/lib/bundle.c#L178

Can you explain it?