Re: [PATCH v2 net] net: fix deadlock while clearing neighbor proxy table

[David Miller <davem@davemloft.net>]

From: Wolfgang Bumiller <w.bumiller@proxmox.com>
Date: Thu, 12 Apr 2018 10:46:55 +0200

> When coming from ndisc_netdev_event() in net/ipv6/ndisc.c,
> neigh_ifdown() is called with &nd_tbl, locking this while
> clearing the proxy neighbor entries when eg. deleting an
> interface. Calling the table's pndisc_destructor() with the
> lock still held, however, can cause a deadlock: When a
> multicast listener is available an IGMP packet of type
> ICMPV6_MGM_REDUCTION may be sent out. When reaching
> ip6_finish_output2(), if no neighbor entry for the target
> address is found, __neigh_create() is called with &nd_tbl,
> which it'll want to lock.
> 
> Move the elements into their own list, then unlock the table
> and perform the destruction.
> 
> Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=199289
> Fixes: 6fd6ce2056de ("ipv6: Do not depend on rt->n in ip6_finish_output2().")
> Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
> ---
> Changes to v1:
>   * Renamed 'pneigh_ifdown' to 'pneigh_ifdown_and_unlock'.

Applied and queued up for -stable.

> Btw. I'm not actually sure how much sense the Fixes tag makes as the
> commit itself isn't wrong, it just happens to be the most easily
> triggerable code path there (and I can't definitively rule out others,
> given that the "sending something over the network with the lock held
> will deadlock" comment at the top of the file is also from the initial
> commit I'd expect other backtraces to be possible from out of that
> function) and the other affected lines are mostly the initial git
> commit...

Understood.