CVE-2021-47069

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

ipc/mqueue, msg, sem: avoid relying on a stack reference past its expiry

do_mq_timedreceive calls wq_sleep with a stack local address. The
sender (do_mq_timedsend) uses this address to later call pipelined_send.

This leads to a very hard to trigger race where a do_mq_timedreceive
call might return and leave do_mq_timedsend to rely on an invalid
address, causing the following crash:

RIP: 0010:wake_q_add_safe+0x13/0x60
Call Trace:
__x64_sys_mq_timedsend+0x2a9/0x490
do_syscall_64+0x80/0x680
entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x7f5928e40343

The race occurs as:

1. do_mq_timedreceive calls wq_sleep with the address of struct ext_wait_queue on function stack (aliased as ewq_addr here) - it
   holds a valid struct ext_wait_queue * as long as the stack has not
   been overwritten.

2. ewq_addr gets added to info->e_wait_q[RECV].list in wq_add, and
   do_mq_timedsend receives it via wq_get_first_waiter(info, RECV) to call
   __pipelined_op.

3. Sender calls __pipelined_op::smp_store_release(&this->state,
   STATE_READY). Here is where the race window begins. (this is
   ewq_addr.)

4. If the receiver wakes up now in do_mq_timedreceive::wq_sleep, it
   will see state == STATE_READY and break.

5. do_mq_timedreceive returns, and ewq_addr is no longer guaranteed
   to be a struct ext_wait_queue * since it was on do_mq_timedreceive’s
   stack. (Although the address may not get overwritten until another
   function happens to touch it, which means it can persist around for an
   indefinite time.)

6. do_mq_timedsend::__pipelined_op() still believes ewq_addr is a
   struct ext_wait_queue *, and uses it to find a task_struct to pass to
   the wake_q_add_safe call. In the lucky case where nothing has
   overwritten ewq_addr yet, ewq_addr->task is the right task_struct.
   In the unlucky case, __pipelined_op::wake_q_add_safe gets handed a
   bogus address as the receiver’s task_struct causing the crash.

do_mq_timedsend::__pipelined_op() should not dereference this after
setting STATE_READY, as the receiver counterpart is now free to return.
Change __pipelined_op to call wake_q_add_safe on the receiver’s
task_struct returned by get_task_struct, instead of dereferencing this
which sits on the receiver’s stack.

As Manfred pointed out, the race potentially also exists in
ipc/msg.c::expunge_all and ipc/sem.c::wake_up_sem_queue_prepare. Fix
those in the same way.