Re: [PATCH v3] bridge: Fix a deadlock when enabling multicast snooping

[Jakub Kicinski]

On Sat, 5 Dec 2020 10:56:45 +0200 Nikolay Aleksandrov wrote:
> On 05/12/2020 01:56, Joseph Huang wrote:
> > When enabling multicast snooping, bridge module deadlocks on multicast_lock
> > if 1) IPv6 is enabled, and 2) there is an existing querier on the same L2
> > network.
> > 
> > The deadlock was caused by the following sequence: While holding the lock,
> > br_multicast_open calls br_multicast_join_snoopers, which eventually causes
> > IP stack to (attempt to) send out a Listener Report (in igmp6_join_group).
> > Since the destination Ethernet address is a multicast address, br_dev_xmit
> > feeds the packet back to the bridge via br_multicast_rcv, which in turn
> > calls br_multicast_add_group, which then deadlocks on multicast_lock.
> > 
> > The fix is to move the call br_multicast_join_snoopers outside of the
> > critical section. This works since br_multicast_join_snoopers only deals
> > with IP and does not modify any multicast data structures of the bridge,
> > so there's no need to hold the lock.
> > 
> > Steps to reproduce:
> > 1. sysctl net.ipv6.conf.all.force_mld_version=1
> > 2. have another querier
> > 3. ip link set dev bridge type bridge mcast_snooping 0 && \
> >    ip link set dev bridge type bridge mcast_snooping 1 < deadlock >
> > 
> > A typical call trace looks like the following:

> > Fixes: 4effd28c1245 ("bridge: join all-snoopers multicast address")
> > Signed-off-by: Joseph Huang <Joseph.Huang@xxxxxxxxxx>
> 
> LGTM, thanks!
> Acked-by: Nikolay Aleksandrov <nikolay@xxxxxxxxxx>

Applied, thank you!