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

From: Petr Machata <[email-masked]>
Date: Thu, 25 Jul 2024 14:42:23 +0200

Eric Dumazet <[email-masked]> writes:

> On Wed, Jul 24, 2024 at 12:09 PM Petr Machata <[email-masked]> wrote:
>>
>>
>> Eric Dumazet <[email-masked]> writes:
>>
>> > On Tue, Jul 23, 2024 at 7:41 PM Eric Dumazet <[email-masked]> wrote:
>> >>
>> >> On Tue, Jul 23, 2024 at 7:26 PM Eric Dumazet <[email-masked]> wrote:
>> >> >
>> >> > On Tue, Jul 23, 2024 at 6:50 PM Eric Dumazet <[email-masked]> wrote:
>> >> > >
>> >> > > On Tue, Jul 23, 2024 at 6:05 PM Petr Machata <[email-masked]> 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 <[email-masked]>
>> >> > > > Reviewed-by: Ido Schimmel <[email-masked]>
>> >> > >
>> >> > > 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)

But that would involve a splat. There's no splat with this issue, even
though I'm testing on a CONFIG_HAVE_ARCH_KMSAN kernel.