CVE-2021-47371

In the Linux kernel, the following vulnerability has been resolved:

nexthop: Fix memory leaks in nexthop notification chain listeners

syzkaller discovered memory leaks [1] that can be reduced to the following commands:

# ip nexthop add id 1 blackhole # devlink dev reload pci/0000:06:00.0

As part of the reload flow, mlxsw will unregister its netdevs and then unregister from the nexthop notification chain. Before unregistering from the notification chain, mlxsw will receive delete notifications for nexthop objects using netdevs registered by mlxsw or their uppers. mlxsw will not receive notifications for nexthops using netdevs that are not dismantled as part of the reload flow. For example, the blackhole nexthop above that internally uses the loopback netdev as its nexthop device.

One way to fix this problem is to have listeners flush their nexthop tables after unregistering from the notification chain. This is error-prone as evident by this patch and also not symmetric with the registration path where a listener receives a dump of all the existing nexthops.

Therefore, fix this problem by replaying delete notifications for the listener being unregistered. This is symmetric to the registration path and also consistent with the netdev notification chain.

The above means that unregisternexthopnotifier(), like registernexthopnotifier(), will have to take RTNL in order to iterate over the existing nexthops and that any callers of the function cannot hold RTNL. This is true for mlxsw and netdevsim, but not for the VXLAN driver. To avoid a deadlock, change the latter to unregister its nexthop listener without holding RTNL, making it symmetric to the registration path.

[1] unreferenced object 0xffff88806173d600 (size 512): comm "syz-executor.0", pid 1290, jiffies 4295583142 (age 143.507s) hex dump (first 32 bytes): 41 9d 1e 60 80 88 ff ff 08 d6 73 61 80 88 ff ff A..`......sa.... 08 d6 73 61 80 88 ff ff 01 00 00 00 00 00 00 00 ..sa............ backtrace: [<ffffffff81a6b576>] kmemleakallocrecursive include/linux/kmemleak.h:43 [inline] [<ffffffff81a6b576>] slabpostallochook+0x96/0x490 mm/slab.h:522 [<ffffffff81a716d3>] slaballocnode mm/slub.c:3206 [inline] [<ffffffff81a716d3>] slaballoc mm/slub.c:3214 [inline] [<ffffffff81a716d3>] kmemcachealloctrace+0x163/0x370 mm/slub.c:3231 [<ffffffff82e8681a>] kmalloc include/linux/slab.h:591 [inline] [<ffffffff82e8681a>] kzalloc include/linux/slab.h:721 [inline] [<ffffffff82e8681a>] mlxswspnexthopobjgroupcreate drivers/net/ethernet/mellanox/mlxsw/spectrumrouter.c:4918 [inline] [<ffffffff82e8681a>] mlxswspnexthopobjnew drivers/net/ethernet/mellanox/mlxsw/spectrumrouter.c:5054 [inline] [<ffffffff82e8681a>] mlxswspnexthopobjevent+0x59a/0x2910 drivers/net/ethernet/mellanox/mlxsw/spectrumrouter.c:5239 [<ffffffff813ef67d>] notifiercallchain+0xbd/0x210 kernel/notifier.c:83 [<ffffffff813f0662>] blockingnotifiercallchain kernel/notifier.c:318 [inline] [<ffffffff813f0662>] blockingnotifiercallchain+0x72/0xa0 kernel/notifier.c:306 [<ffffffff8384b9c6>] callnexthopnotifiers+0x156/0x310 net/ipv4/nexthop.c:244 [<ffffffff83852bd8>] insertnexthop net/ipv4/nexthop.c:2336 [inline] [<ffffffff83852bd8>] nexthopadd net/ipv4/nexthop.c:2644 [inline] [<ffffffff83852bd8>] rtmnewnexthop+0x14e8/0x4d10 net/ipv4/nexthop.c:2913 [<ffffffff833e9a78>] rtnetlinkrcvmsg+0x448/0xbf0 net/core/rtnetlink.c:5572 [<ffffffff83608703>] netlinkrcvskb+0x173/0x480 net/netlink/afnetlink.c:2504 [<ffffffff833de032>] rtnetlinkrcv+0x22/0x30 net/core/rtnetlink.c:5590 [<ffffffff836069de>] netlinkunicastkernel net/netlink/afnetlink.c:1314 [inline] [<ffffffff836069de>] netlinkunicast+0x5ae/0x7f0 net/netlink/afnetlink.c:1340 [<ffffffff83607501>] netlinksendmsg+0x8e1/0xe30 net/netlink/afnetlink.c:1929 [<ffffffff832fde84>] socksendmsgnosec net/socket.c:704 [inline ---truncated---