[PATCH v2] ipc/mqueue: Avoid relying on a stack reference past its expiry

[Varad Gautam]

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:

[  240.739977] RIP: 0010:wake_q_add_safe+0x13/0x60
[  240.739991] Call Trace:
[  240.739999]  __x64_sys_mq_timedsend+0x2a9/0x490
[  240.740003]  ? auditd_test_task+0x38/0x40
[  240.740007]  ? auditd_test_task+0x38/0x40
[  240.740011]  do_syscall_64+0x80/0x680
[  240.740017]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[  240.740019] 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. `ewq_addr` gets removed from
info->e_wait_q[RECV].list.

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 before setting STATE_READY
which ensures that the receiver's task_struct can still be found via
`this`.

Fixes: c5b2cbdbdac563 ("ipc/mqueue.c: update/document memory barriers")
Signed-off-by: Varad Gautam <varad.gautam@xxxxxxxx>
Reported-by: Matthias von Faber <matthias.vonfaber@xxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # 5.6
Cc: Christian Brauner <christian.brauner@xxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Cc: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Davidlohr Bueso <dbueso@xxxxxxx>
---
v2: Call wake_q_add before smp_store_release, instead of using a
    get_task_struct/wake_q_add_safe combination across
    smp_store_release. (Davidlohr Bueso)

 ipc/mqueue.c | 33 ++++++++++++++++++++++++---------
 1 file changed, 24 insertions(+), 9 deletions(-)

diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index 8031464ed4ae..bfcb6f81a824 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -78,11 +78,13 @@ struct posix_msg_tree_node {
  * MQ_BARRIER:
  * To achieve proper release/acquire memory barrier pairing, the state is set to
  * STATE_READY with smp_store_release(), and it is read with READ_ONCE followed
- * by smp_acquire__after_ctrl_dep(). In addition, wake_q_add_safe() is used.
+ * by smp_acquire__after_ctrl_dep(). The state change to STATE_READY must be
+ * the last write operation, after which the blocked task can immediately
+ * return and exit.
  *
  * This prevents the following races:
  *
- * 1) With the simple wake_q_add(), the task could be gone already before
+ * 1) With wake_q_add(), the task could be gone already before
  *    the increase of the reference happens
  * Thread A
  *				Thread B
@@ -97,10 +99,25 @@ struct posix_msg_tree_node {
  * sys_exit()
  *				get_task_struct() // UaF
  *
- * Solution: Use wake_q_add_safe() and perform the get_task_struct() before
- * the smp_store_release() that does ->state = STATE_READY.
+ * 2) With wake_q_add(), the receiver task could have returned from the
+ *    syscall and had its stack-allocated waiter overwritten before the
+ *    sender could add it to the wake_q
+ * Thread A
+ *				Thread B
+ * WRITE_ONCE(wait.state, STATE_NONE);
+ * schedule_hrtimeout()
+ *				->state = STATE_READY
+ * <timeout returns>
+ * if (wait.state == STATE_READY) return;
+ * sysret to user space
+ * overwrite receiver's stack
+ *				wake_q_add(A)
+ *				if (cmpxchg()) // corrupted waiter
  *
- * 2) Without proper _release/_acquire barriers, the woken up task
+ * Solution: Queue the task for wakeup before the smp_store_release() that
+ * does ->state = STATE_READY.
+ *
+ * 3) Without proper _release/_acquire barriers, the woken up task
  *    could read stale data
  *
  * Thread A
@@ -116,7 +133,7 @@ struct posix_msg_tree_node {
  *
  * Solution: use _release and _acquire barriers.
  *
- * 3) There is intentionally no barrier when setting current->state
+ * 4) There is intentionally no barrier when setting current->state
  *    to TASK_INTERRUPTIBLE: spin_unlock(&info->lock) provides the
  *    release memory barrier, and the wakeup is triggered when holding
  *    info->lock, i.e. spin_lock(&info->lock) provided a pairing
@@ -1005,11 +1022,9 @@ static inline void __pipelined_op(struct wake_q_head *wake_q,
 				  struct ext_wait_queue *this)
 {
 	list_del(&this->list);
-	get_task_struct(this->task);
-
+	wake_q_add(wake_q, this->task);
 	/* see MQ_BARRIER for purpose/pairing */
 	smp_store_release(&this->state, STATE_READY);
-	wake_q_add_safe(wake_q, this->task);
 }
 
 /* pipelined_send() - send a message directly to the task waiting in
-- 
2.30.2