netdev - Re: [PATCH net-next v2] net: dccp: initialize (addr,port) listening hashtable

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Date:   Sun, 16 Dec 2018 22:35:16 -0800
From:   Peter Oskolkov <posk.devel@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Peter Oskolkov <posk@...gle.com>,
        David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        Eric Dumazet <edumazet@...gle.com>,
        syzcaller <syzkaller@...glegroups.com>,
        Martin KaFai Lau <kafai@...com>
Subject: Re: [PATCH net-next v2] net: dccp: initialize (addr,port) listening hashtable

I guess DCCP is not used to the extent that 10 listeners per bucket
happen in real life scenarios, so the issue is purely theoretical; as
my patch would cause a sefgault for even a single listening dccp
socket, I felt it was my patch that the "Fixes" tag should refer to.

On Sun, Dec 16, 2018 at 9:51 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
>
> On 12/16/2018 03:42 PM, Peter Oskolkov wrote:
> > Commit d9fbc7f6431f "net: tcp: prefer listeners bound to an address"
> > removes port-only listener lookups. This caused segfaults in DCCP
> > lookups because DCCP did not initialize the (addr,port) hashtable.
> >
> > This patch adds said initialization.
> >
> > The only non-trivial issue here is the size of the new hashtable.
> > It seemed reasonable to make it match the size of the port-only
> > hashtable (= INET_LHTABLE_SIZE) that was used previously. Other
> > parameters to inet_hashinfo2_init() match those used in TCP.
> >
> > V2 changes: marked inet_hashinfo2_init as an exported symbol
> > so that DCCP compiles when configured as a module.
> >
> > Tested: syzcaller issues fixed; the second patch in the patchset
> >         tests that DCCP lookups work correctly.
> >
> > Fixes: d9fbc7f6431f "net: tcp: prefer listeners bound to an address"
>
>
> Strange, I would say bug was brought by commit 61b7c691c731
> ("inet: Add a 2nd listener hashtable (port+addr)")
>
> The secondary hash table would have been used anyway if some application (syzkaller-like)
> had been using DCCP a bit more, to the point the second hash table would have been used.
>
> With more than 10 listeners on one bucket, code in inet6_lookup_listener() would
> have attempted to deref a NULL pointer.