CVE-2022-49998 — rxrpc: Fix locking in rxrpc's sendmsg

Affected: Linux

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

rxrpc: Fix locking in rxrpc's sendmsg

Fix three bugs in the rxrpc's sendmsg implementation:

(1) rxrpc_new_client_call() should release the socket lock when returning an error from rxrpc_get_call_slot().

(2) rxrpc_wait_for_tx_window_intr() will return without the call mutex held in the event that we're interrupted by a signal whilst waiting for tx space on the socket or relocking the call mutex afterwards.

Fix this by: (a) moving the unlock/lock of the call mutex up to rxrpc_send_data() such that the lock is not held around all of rxrpc_wait_for_tx_window*() and (b) indicating to higher callers whether we're return with the lock dropped. Note that this means recvmsg() will not block on this call whilst we're waiting.

(3) After dropping and regaining the call mutex, rxrpc_send_data() needs to go and recheck the state of the tx_pending buffer and the tx_total_len check in case we raced with another sendmsg() on the same call.

Thinking on this some more, it might make sense to have different locks for sendmsg() and recvmsg(). There's probably no need to make recvmsg() wait for sendmsg(). It does mean that recvmsg() can return MSG_EOR indicating that a call is dead before a sendmsg() to that call returns - but that can currently happen anyway.

Without fix (2), something like the following can be induced:

WARNING: bad unlock balance detected!…

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CVE-2022-49998 — rxrpc: Fix locking in rxrpc's sendmsg