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

[Nikolay Aleksandrov <nikolay@nvidia.com>]

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:
> 
> [  936.251495]  _raw_spin_lock+0x5c/0x68
> [  936.255221]  br_multicast_add_group+0x40/0x170 [bridge]
> [  936.260491]  br_multicast_rcv+0x7ac/0xe30 [bridge]
> [  936.265322]  br_dev_xmit+0x140/0x368 [bridge]
> [  936.269689]  dev_hard_start_xmit+0x94/0x158
> [  936.273876]  __dev_queue_xmit+0x5ac/0x7f8
> [  936.277890]  dev_queue_xmit+0x10/0x18
> [  936.281563]  neigh_resolve_output+0xec/0x198
> [  936.285845]  ip6_finish_output2+0x240/0x710
> [  936.290039]  __ip6_finish_output+0x130/0x170
> [  936.294318]  ip6_output+0x6c/0x1c8
> [  936.297731]  NF_HOOK.constprop.0+0xd8/0xe8
> [  936.301834]  igmp6_send+0x358/0x558
> [  936.305326]  igmp6_join_group.part.0+0x30/0xf0
> [  936.309774]  igmp6_group_added+0xfc/0x110
> [  936.313787]  __ipv6_dev_mc_inc+0x1a4/0x290
> [  936.317885]  ipv6_dev_mc_inc+0x10/0x18
> [  936.321677]  br_multicast_open+0xbc/0x110 [bridge]
> [  936.326506]  br_multicast_toggle+0xec/0x140 [bridge]
> 
> Fixes: 4effd28c1245 ("bridge: join all-snoopers multicast address")
> Signed-off-by: Joseph Huang <Joseph.Huang@garmin.com>
> ---
>  net/bridge/br_device.c    |  6 ++++++
>  net/bridge/br_multicast.c | 34 +++++++++++++++++++++++++---------
>  net/bridge/br_private.h   | 10 ++++++++++
>  3 files changed, 41 insertions(+), 9 deletions(-)
> 

LGTM, thanks!
Acked-by: Nikolay Aleksandrov <nikolay@nvidia.com>