CVE-2022-49998

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) rxrpcnewclientcall() should release the socket lock when returning an error from rxrpcgetcallslot().

(2) rxrpcwaitfortxwindow_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, rxrpcsenddata() needs to go and recheck the state of the txpending buffer and the txtotal_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!
5.16.0-rc6-syzkaller #0 Not tainted
-------------------------------------
syz-executor011/3597 is trying to release lock (&call->user_mutex) at:
[<ffffffff885163a3>] rxrpc_do_sendmsg+0xc13/0x1350 net/rxrpc/sendmsg.c:748
but there are no more locks to release!

other info that might help us debug this:
no locks held by syz-executor011/3597.
...
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
 print_unlock_imbalance_bug include/trace/events/lock.h:58 [inline]
 __lock_release kernel/locking/lockdep.c:5306 [inline]
 lock_release.cold+0x49/0x4e kernel/locking/lockdep.c:5657
 __mutex_unlock_slowpath+0x99/0x5e0 kernel/locking/mutex.c:900
 rxrpc_do_sendmsg+0xc13/0x1350 net/rxrpc/sendmsg.c:748
 rxrpc_sendmsg+0x420/0x630 net/rxrpc/af_rxrpc.c:561
 sock_sendmsg_nosec net/socket.c:704 [inline]
 sock_sendmsg+0xcf/0x120 net/socket.c:724
 ____sys_sendmsg+0x6e8/0x810 net/socket.c:2409
 ___sys_sendmsg+0xf3/0x170 net/socket.c:2463
 __sys_sendmsg+0xe5/0x1b0 net/socket.c:2492
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

[Thanks to Hawkins Jiawei and Khalid Masum for their attempts to fix this]