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

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Date:   Sun, 16 Dec 2018 22:49:57 -0800
From:   Peter Oskolkov <posk.devel@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Peter Oskolkov <posk@...gle.com>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        syzbot <syzkaller@...glegroups.com>,
        Martin KaFai Lau <kafai@...com>
Subject: Re: [PATCH net-next v2] net: dccp: initialize (addr,port) listening hashtable

On Sun, Dec 16, 2018 at 10:46 PM Peter Oskolkov <posk.devel@...il.com> wrote:
>
> On Sun, Dec 16, 2018 at 10:41 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > On Sun, Dec 16, 2018 at 10:35 PM Peter Oskolkov <posk.devel@...il.com> wrote:
> > >
> > > 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.
> > >
> >
> > Problem is : any host with a buggy kernel can trivially crash if a
> > malicious application
> > knows about this bug and exploit it.
> >
> > This means we need to backport this fix to all kernels up to the real
> > root cause,
> > not only net-next on which your patch revealed this old bug.
> >
> > Therefore we need a correct Fixes: tag so that proper stable
> > submissions can be done.
> >
> > The Fixes: tag accuracy is paramount.
>
> Make sense. I'll re-send the patch withe the correct Fixes tag asap.

Actually, no: the earlier patch has this condition:

+       if (ilb->count <= 10 || !hashinfo->lhash2)
+               goto port_lookup;

that checks if lhash2 is initialized. So the bug is in my patch.

>
> >
> > Thanks.
> >
> > > 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.