bpf: devmap fix mutex in rcu critical section

Originally we used a mutex to protect concurrent devmap update
and delete operations from racing with netdev unregister notifier
callbacks.

The notifier hook is needed because we increment the netdev ref
count when a dev is added to the devmap. This ensures the netdev
reference is valid in the datapath. However, we don't want to block
unregister events, hence the initial mutex and notifier handler.

The concern was in the notifier hook we search the map for dev
entries that hold a refcnt on the net device being torn down. But,
in order to do this we require two steps,

  (i) dereference the netdev:  dev = rcu_dereference(map[i])
 (ii) test ifindex:   dev->ifindex == removing_ifindex

and then finally we can swap in the NULL dev in the map via an
xchg operation,

  xchg(map[i], NULL)

The danger here is a concurrent update could run a different
xchg op concurrently leading us to replace the new dev with a
NULL dev incorrectly.

      CPU 1                        CPU 2

   notifier hook                   bpf devmap update

   dev = rcu_dereference(map[i])
                                   dev = rcu_dereference(map[i])
                                   xchg(map[i]), new_dev);
                                   rcu_call(dev,...)
   xchg(map[i], NULL)

The above flow would create the incorrect state with the dev
reference in the update path being lost. To resolve this the
original code used a mutex around the above block. However,
updates, deletes, and lookups occur inside rcu critical sections
so we can't use a mutex in this context safely.

Fortunately, by writing slightly better code we can avoid the
mutex altogether. If CPU 1 in the above example uses a cmpxchg
and _only_ replaces the dev reference in the map when it is in
fact the expected dev the race is removed completely. The two
cases being illustrated here, first the race condition,

      CPU 1                          CPU 2

   notifier hook                     bpf devmap update

   dev = rcu_dereference(map[i])
                                     dev = rcu_dereference(map[i])
                                     xchg(map[i]), new_dev);
                                     rcu_call(dev,...)
   odev = cmpxchg(map[i], dev, NULL)

Now we can test the cmpxchg return value, detect odev != dev and
abort. Or in the good case,

      CPU 1                          CPU 2

   notifier hook                     bpf devmap update
   dev = rcu_dereference(map[i])
   odev = cmpxchg(map[i], dev, NULL)
                                     [...]

Now 'odev == dev' and we can do proper cleanup.

And viola the original race we tried to solve with a mutex is
corrected and the trace noted by Sasha below is resolved due
to removal of the mutex.

Note: When walking the devmap and removing dev references as needed
we depend on the core to fail any calls to dev_get_by_index() using
the ifindex of the device being removed. This way we do not race with
the user while searching the devmap.

Additionally, the mutex was also protecting list add/del/read on
the list of maps in-use. This patch converts this to an RCU list
and spinlock implementation. This protects the list from concurrent
alloc/free operations. The notifier hook walks this list so it uses
RCU read semantics.

BUG: sleeping function called from invalid context at kernel/locking/mutex.c:747
in_atomic(): 1, irqs_disabled(): 0, pid: 16315, name: syz-executor1
1 lock held by syz-executor1/16315:
 #0:  (rcu_read_lock){......}, at: [<ffffffff8c363bc2>] map_delete_elem kernel/bpf/syscall.c:577 [inline]
 #0:  (rcu_read_lock){......}, at: [<ffffffff8c363bc2>] SYSC_bpf kernel/bpf/syscall.c:1427 [inline]
 #0:  (rcu_read_lock){......}, at: [<ffffffff8c363bc2>] SyS_bpf+0x1d32/0x4ba0 kernel/bpf/syscall.c:1388

Fixes: 2ddf71e2 ("net: add notifier hooks for devmap bpf map")
Reported-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>