AZL-97814

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

afunix: Unlink sccentry in unixdeledge().

Kyle Zeng reported that GC could free a dead SCC partially.

The scenario is as follows:

1) Create two SCCs:

   X -.   A <-> B
   ^--'

2) Run the following concurrently:

  2-1) send() sk-B to sk-B from sk-X
  2-2) close() both A and B

At 2-1), there is a small window where unixaddedges() publishes a new edge (B <-> B) to GC but its skb is not queued by skbqueuetail().

If 2-2) completes before skbqueuetail() and GC is triggered, it judges A <-> B as dead, but B is not freed because GC cannot collect the not-yet-queued skb holding the B <-> B edge.

   X -.   A <-> B -. This edge is visible
   ^--'         ^..'  but skb is not

This itself is not a problem since the next GC run will judge B as dead as well and free it finally.

   X -.   A <.> B -.
   ^--'         ^--'

However, X's SCC forces the next GC to call unixwalksccfast(), and it iterates over A through B's sccentry.

Let's unlink sccentry before freeing the vertex in unixdel_edge().