netdev - Re: [PATCH net] net: nexthop: Initialize all fields in dumped nexthops

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Message-ID: <CANn89iJ6vG3n0bUbGuHosRDwW97z7CT4m4+_D91onPK5rd8xVw@mail.gmail.com>
Date: Tue, 23 Jul 2024 19:41:08 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Petr Machata <petrm@...dia.com>
Cc: "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, 
	David Ahern <dsahern@...nel.org>, mlxsw@...dia.com, Ido Schimmel <idosch@...dia.com>
Subject: Re: [PATCH net] net: nexthop: Initialize all fields in dumped nexthops

On Tue, Jul 23, 2024 at 7:26 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Tue, Jul 23, 2024 at 6:50 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > On Tue, Jul 23, 2024 at 6:05 PM Petr Machata <petrm@...dia.com> wrote:
> > >
> > > struct nexthop_grp contains two reserved fields that are not initialized by
> > > nla_put_nh_group(), and carry garbage. This can be observed e.g. with
> > > strace (edited for clarity):
> > >
> > >     # ip nexthop add id 1 dev lo
> > >     # ip nexthop add id 101 group 1
> > >     # strace -e recvmsg ip nexthop get id 101
> > >     ...
> > >     recvmsg(... [{nla_len=12, nla_type=NHA_GROUP},
> > >                  [{id=1, weight=0, resvd1=0x69, resvd2=0x67}]] ...) = 52
> > >
> > > The fields are reserved and therefore not currently used. But as they are, they
> > > leak kernel memory, and the fact they are not just zero complicates repurposing
> > > of the fields for new ends. Initialize the full structure.
> > >
> > > Fixes: 430a049190de ("nexthop: Add support for nexthop groups")
> > > Signed-off-by: Petr Machata <petrm@...dia.com>
> > > Reviewed-by: Ido Schimmel <idosch@...dia.com>
> >
> > Interesting... not sure why syzbot did not catch this one.
> >
> > Reviewed-by: Eric Dumazet <edumazet@...gle.com>
>
> Hmmm... Do we have the guarantee that the compiler initializes padding ?
>
> AFAIK, padding at the end of the structure is not initialized.

I am asking this because syzbot found a similar issue in net/sched/act_ct.c

My current WIP is :

diff --git a/net/sched/act_ct.c b/net/sched/act_ct.c
index 113b907da0f757af7be920cc9b3a1b1c769f5804..3ba8e7e739b58a96e66ca64d38bff758500df3e1
100644
--- a/net/sched/act_ct.c
+++ b/net/sched/act_ct.c
@@ -44,6 +44,8 @@ static DEFINE_MUTEX(zones_mutex);
 struct zones_ht_key {
        struct net *net;
        u16 zone;
+       /* Note : pad[] must be the last field. */
+       u8  pad[];
 };

 struct tcf_ct_flow_table {
@@ -60,7 +62,7 @@ struct tcf_ct_flow_table {
 static const struct rhashtable_params zones_params = {
        .head_offset = offsetof(struct tcf_ct_flow_table, node),
        .key_offset = offsetof(struct tcf_ct_flow_table, key),
-       .key_len = sizeof_field(struct tcf_ct_flow_table, key),
+       .key_len = offsetof(struct zones_ht_key, pad),
        .automatic_shrinking = true,
 };