Re: [PATCH net 5/6] net/diag: Limit TCP-MD5-diag array by max attribute length

[Dmitry Safonov]

On Thu, 7 Nov 2024 at 00:26, Kuniyuki Iwashima <kuniyu@xxxxxxxxxx> wrote:
>
> From: Dmitry Safonov via B4 Relay <devnull+0x7f454c46.gmail.com@xxxxxxxxxx>
> Date: Wed, 06 Nov 2024 18:10:18 +0000
> > From: Dmitry Safonov <0x7f454c46@xxxxxxxxx>
> >
> > Currently TCP-MD5 keys are dumped as an array of
> > (struct tcp_diag_md5sig). All the keys from a socket go
> > into the same netlink attribute. The maximum amount of TCP-MD5 keys on
> > any socket is limited by /proc/sys/net/core/optmem_max, which post
> > commit 4944566706b2 ("net: increase optmem_max default value") is now by
> > default 128 KB. With the help of selftest I've figured out that equals
> > to 963 keys, without user having to increase optmem_max:
> > > test_set_md5() [963/1024]: Cannot allocate memory
> >
> > The maximum length of nlattr is limited by typeof(nlattr::nla_len),
> > which is (U16_MAX - 1). When there are too many keys the array written
> > overflows the netlink attribute. Here is what one can see on a test,
> > with no adjustments to optmem_max defaults:
> >
> > > recv() = 65180
> > > socket: 10.0.254.1:7013->0.0.0.0:0 (intf 3)
> > >      family: 2 state: 10 timer: 0 retrans: 0
> > >      expires: 0 rqueu: 0 wqueue: 1 uid: 0 inode: 456
> > >              attr type: 8 (5)
> > >              attr type: 15 (8)
> > >              attr type: 21 (12)
> > >              attr type: 22 (6)
> > >              attr type: 2 (252)
> > >              attr type: 18 (64804)
> > > recv() = 130680
> > > socket: 10.0.254.1:7013->0.0.0.0:0 (intf 3)
> > >      family: 2 state: 10 timer: 0 retrans: 0
> > >      expires: 0 rqueu: 0 wqueue: 1 uid: 0 inode: 456
> > >              attr type: 8 (5)
> > >              attr type: 15 (8)
> > >              attr type: 21 (12)
> > >              attr type: 22 (6)
> > >              attr type: 2 (252)
> > >              attr type: 18 (64768)
> > >              attr type: 29555 (25966)
> > > recv() = 130680
> > > socket: 10.0.254.1:7013->0.0.0.0:0 (intf 3)
> > >      family: 2 state: 10 timer: 0 retrans: 0
> > >      expires: 0 rqueu: 0 wqueue: 1 uid: 0 inode: 456
> > >              attr type: 8 (5)
> > >              attr type: 15 (8)
> > >              attr type: 21 (12)
> > >              attr type: 22 (6)
> > >              attr type: 2 (252)
> > >              attr type: 18 (64768)
> > >              attr type: 29555 (25966)
> > >              attr type: 8265 (8236)
> >
> > Here attribute type 18 is INET_DIAG_MD5SIG, the following nlattr types
> > are junk made of tcp_diag_md5sig's content.
> >
> > Here is the overflow of the nlattr size:
> > >>> hex(64768)
> > '0xfd00'
> > >>> hex(130300)
> > '0x1fcfc'
> >
> > Limit the size of (struct tcp_diag_md5sig) array in the netlink reply by
> > maximum attribute length. Not perfect as NLM_F_DUMP_INTR will be set on
> > the netlink header flags, but the userspace can differ if it's due to
> > inconsistency or due to maximum size of the netlink attribute.
> >
> > In a following patch set, I'm planning to address this and re-introduce
> > TCP-MD5-diag that actually works.
>
> Given the issue has not been reported so far (I think), we can wait for
> the series rather than backporting this.

Yeah, my concern is that ss or or other tools may interrupt the
md5keys from overflow as other netlink attributes and either show
non-meaningful things or even hide some socket information (as if an
attribute is met the second time, from what I read in ss code, it will
put the last met attribute of the same type over previous pointers and
print only that).

Regarding reports, one has to have U16_MAX / sizeof(struct
tcp_diag_md5sig) = 655 keys on a socket. Probably, not many people use
BGP with that many peers.

I'm fine moving that to later net-next patches; I've sent it only
because the above seemed concerning to me.

Thanks,
             Dmitry