In the Linux kernel, the following vulnerability has been resolved: net/rds: acquire RDS_IN_XMIT in rds_tcp_reset_callbacks() rds_tcp_rese

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

net/rds: acquire RDS_IN_XMIT in rds_tcp_reset_callbacks()

rds_tcp_reset_callbacks() quiesces the transmit path by setting the
path state to RDS_CONN_RESETTING and then waiting for RDS_IN_XMIT to
be sampled clear before swapping the underlying socket and calling
rds_send_path_reset().

Sampling the bit clear is not the same as owning it: rds_send_xmit()
can re-acquire RDS_IN_XMIT right after the wait_event() returns. Its
state recheck after taking the lock is a store-buffering pattern (the
resetter writes the state and reads the bit, the sender writes the
bit and reads the state) and acquire_in_xmit() is only an acquire
operation, so on weakly ordered architectures both sides can miss
each other's write and the transmit path then runs concurrently with
rds_send_path_reset() rewriting cp_xmit_* state - which is exactly
what the comment above rds_send_path_reset() tells its callers to
prevent.

Take the lock instead, hold it across the socket swap and
rds_send_path_reset(), and release it with a wake-up at the end. The
lock-ordering constraint documented above the wait still holds: the
lock is acquired before lock_sock(), so a sender inside tcp_sendmsg()
can never be waited on while we hold the socket lock.

Two details of the old code go away with the same change:

• t_sock is now read only after the lock is acquired. The old code
  cached it before waiting; the teardown in rds_conn_shutdown()
  releases that socket and clears t_sock, so a pointer cached before
  the wait can be stale by the time the accept path resumes. Reading
  it under RDS_IN_XMIT is what makes the exclusion complete once the
  teardown owns the same lock, which the next patch arranges; until
  then the teardown still only samples the bit, and the two paths
  remain as exposed to each other as they are today.

• The old !osock early path called rds_send_path_reset() with no
  serialization at all. It now runs under the lock like the normal
  path. The conditional RDS_CONN_RESETTING transition of the
  previous patch happens before the socket check either way: a path
  found without a socket is either still connecting (its reconnect
  worker blocked on t_conn_path_lock) and legitimately goes
  RESETTING -> UP on the new socket, or it has been torn down
  meanwhile and is dropped.

The in-function comment describing the old wait-based quiesce is
rewritten to describe the lock-based one, and the stale block comment
above the function (which still described a return value and an
incomplete list of t_sock writers) is refreshed to name all four
writers - the connect, accept, teardown and swap paths - and what
serializes each of them.