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

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Message-ID: <CANn89iLGh6sG8AhD_dgb15Es6MsATZ+QHkNzHwm5iufTCXZ+SA@mail.gmail.com>
Date: Wed, 24 Jul 2024 12:36:54 +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 Wed, Jul 24, 2024 at 12:09 PM Petr Machata <petrm@...dia.com> wrote:
>
>
> Eric Dumazet <edumazet@...gle.com> writes:
>
> > On Tue, Jul 23, 2024 at 7:41 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >>
> >> 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.
>
> Could it? I'm not sure of the exact syzcaller capabilities, but there
> are no warnings, no splats etc. It just returns values.

Yes, KMSAN can detect such things (uninit-value)