Re: [PATCH 1/1] net/sched: cls_u32: Fix reference counter leak leading to overflow

[Eric Dumazet]

On Wed, May 31, 2023 at 4:16 PM Lee Jones <lee@xxxxxxxxxx> wrote:
>
> In the event of a failure in tcf_change_indev(), u32_set_parms() will
> immediately return without decrementing the recently incremented
> reference counter.  If this happens enough times, the counter will
> rollover and the reference freed, leading to a double free which can be
> used to do 'bad things'.
>
> Cc: stable@xxxxxxxxxx # v4.14+

Please add a Fixes: tag.

> Signed-off-by: Lee Jones <lee@xxxxxxxxxx>
> ---
>  net/sched/cls_u32.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/net/sched/cls_u32.c b/net/sched/cls_u32.c
> index 4e2e269f121f8..fad61ca5e90bf 100644
> --- a/net/sched/cls_u32.c
> +++ b/net/sched/cls_u32.c
> @@ -762,8 +762,11 @@ static int u32_set_parms(struct net *net, struct tcf_proto *tp,
>         if (tb[TCA_U32_INDEV]) {
>                 int ret;
>                 ret = tcf_change_indev(net, tb[TCA_U32_INDEV], extack);

This call should probably be done earlier in the function, next to
tcf_exts_validate_ex()

Otherwise we might ask why the tcf_bind_filter() does not need to be undone.

Something like:

diff --git a/net/sched/cls_u32.c b/net/sched/cls_u32.c
index 4e2e269f121f8a301368b9783753e055f5af6a4e..ac957ff2216ae18bcabdd3af3b0e127447ef8f91
100644
--- a/net/sched/cls_u32.c
+++ b/net/sched/cls_u32.c
@@ -718,13 +718,18 @@ static int u32_set_parms(struct net *net, struct
tcf_proto *tp,
                         struct nlattr *est, u32 flags, u32 fl_flags,
                         struct netlink_ext_ack *extack)
 {
-       int err;
+       int err, ifindex = -1;

        err = tcf_exts_validate_ex(net, tp, tb, est, &n->exts, flags,
                                   fl_flags, extack);
        if (err < 0)
                return err;

+       if (tb[TCA_U32_INDEV]) {
+               ifindex = tcf_change_indev(net, tb[TCA_U32_INDEV], extack);
+               if (ifindex < 0)
+                       return -EINVAL;
+       }
        if (tb[TCA_U32_LINK]) {
                u32 handle = nla_get_u32(tb[TCA_U32_LINK]);
                struct tc_u_hnode *ht_down = NULL, *ht_old;
@@ -759,13 +764,9 @@ static int u32_set_parms(struct net *net, struct
tcf_proto *tp,
                tcf_bind_filter(tp, &n->res, base);
        }

-       if (tb[TCA_U32_INDEV]) {
-               int ret;
-               ret = tcf_change_indev(net, tb[TCA_U32_INDEV], extack);
-               if (ret < 0)
-                       return -EINVAL;
-               n->ifindex = ret;
-       }
+       if (ifindex >= 0)
+               n->ifindex = ifindex;
+
        return 0;
 }