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From:   Eric Dumazet <edumazet@google.com>
Date:   Wed, 31 May 2023 17:03:37 +0200
Message-ID: <CANn89iJw2N9EbF+Fm8KCPMvo-25ONwba+3PUr8L2ktZC1Z3uLw@mail.gmail.com>
Subject: Re: [PATCH 1/1] net/sched: cls_u32: Fix reference counter leak
 leading to overflow
To:     Lee Jones <lee@kernel.org>
Cc:     jhs@mojatatu.com, xiyou.wangcong@gmail.com, jiri@resnulli.us,
        davem@davemloft.net, kuba@kernel.org, pabeni@redhat.com,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        stable@kernel.org
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On Wed, May 31, 2023 at 4:16=E2=80=AFPM Lee Jones <lee@kernel.org> 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@kernel.org # v4.14+

Please add a Fixes: tag.

> Signed-off-by: Lee Jones <lee@kernel.org>
> ---
>  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 =3D 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..ac957ff2216ae18bcabdd3af3b0=
e127447ef8f91
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 =3D -1;

        err =3D 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 =3D tcf_change_indev(net, tb[TCA_U32_INDEV], extack=
);
+               if (ifindex < 0)
+                       return -EINVAL;
+       }
        if (tb[TCA_U32_LINK]) {
                u32 handle =3D nla_get_u32(tb[TCA_U32_LINK]);
                struct tc_u_hnode *ht_down =3D 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 =3D tcf_change_indev(net, tb[TCA_U32_INDEV], extack);
-               if (ret < 0)
-                       return -EINVAL;
-               n->ifindex =3D ret;
-       }
+       if (ifindex >=3D 0)
+               n->ifindex =3D ifindex;
+
        return 0;
 }