Transfer findings from ipc/sem.c:
- A control barrier was missing for the lockless receive case
So in theory, not yet initialized data may have been copied
to user space - obviously only for architectures where
control barriers are not NOP.
- Add documentation. Especially, document that the code relies
on the barrier inside wake_q_add().
Signed-off-by: Manfred Spraul <manf...@colorfullife.com>
Cc: Waiman Long <long...@redhat.com>
Cc: Davidlohr Bueso <d...@stgolabs.net>
---
ipc/msg.c | 39 ++++++++++++++++++++++++++++++++++++++-
1 file changed, 38 insertions(+), 1 deletion(-)
diff --git a/ipc/msg.c b/ipc/msg.c
index 8dec945fa030..1e2c0a3d4998 100644
--- a/ipc/msg.c
+++ b/ipc/msg.c
@@ -184,6 +184,10 @@ static inline void ss_add(struct msg_queue *msq,
{
mss->tsk = current;
mss->msgsz = msgsz;
+ /*
+ * No memory barrier required: we did ipc_lock_object(),
+ * and the waker obtains that lock before calling wake_q_add().
+ */
__set_current_state(TASK_INTERRUPTIBLE);
list_add_tail(&mss->list, &msq->q_senders);
}
@@ -238,6 +242,12 @@ static void expunge_all(struct msg_queue *msq, int res,
list_for_each_entry_safe(msr, t, &msq->q_receivers, r_list) {
wake_q_add(wake_q, msr->r_tsk);
+
+ /*
+ * A memory barrier is required that pairs with the
+ * READ_ONCE()+smp_mb__after_ctrl_dep(). It is provided by
+ * wake_q_add().
+ */
WRITE_ONCE(msr->r_msg, ERR_PTR(res));
}
}
@@ -798,12 +808,24 @@ static inline int pipelined_send(struct msg_queue *msq,
struct msg_msg *msg,
list_del(&msr->r_list);
if (msr->r_maxsize < msg->m_ts) {
wake_q_add(wake_q, msr->r_tsk);
+
+ /*
+ * A memory barrier is required that pairs with
+ * the READ_ONCE()+smp_mb__after_ctrl_dep().
+ * It is provided by wake_q_add().
+ */
WRITE_ONCE(msr->r_msg, ERR_PTR(-E2BIG));
} else {
ipc_update_pid(&msq->q_lrpid,
task_pid(msr->r_tsk));
msq->q_rtime = ktime_get_real_seconds();
wake_q_add(wake_q, msr->r_tsk);
+
+ /*
+ * A memory barrier is required that pairs with
+ * the READ_ONCE()+smp_mb__after_ctrl_dep().
+ * It is provided by wake_q_add().
+ */
WRITE_ONCE(msr->r_msg, msg);
return 1;
}
@@ -1155,6 +1177,8 @@ static long do_msgrcv(int msqid, void __user *buf, size_t
bufsz, long msgtyp, in
else
msr_d.r_maxsize = bufsz;
msr_d.r_msg = ERR_PTR(-EAGAIN);
+
+ /* memory barrier not required, we own ipc_lock_object() */
__set_current_state(TASK_INTERRUPTIBLE);
ipc_unlock_object(&msq->q_perm);
@@ -1183,8 +1207,21 @@ static long do_msgrcv(int msqid, void __user *buf,
size_t bufsz, long msgtyp, in
* signal) it will either see the message and continue ...
*/
msg = READ_ONCE(msr_d.r_msg);
- if (msg != ERR_PTR(-EAGAIN))
+ if (msg != ERR_PTR(-EAGAIN)) {
+ /*
+ * Memory barrier for msr_d.r_msg
+ * The smp_acquire__after_ctrl_dep(), together with the
+ * READ_ONCE() above pairs with the barrier inside
+ * wake_q_add().
+ * The barrier protects the accesses to the message in
+ * do_msg_fill(). In addition, the barrier protects user
+ * space, too: User space may assume that all data from
+ * the CPU that sent the message is visible.
+ */
+ smp_acquire__after_ctrl_dep();
+
goto out_unlock1;
+ }
/*
* ... or see -EAGAIN, acquire the lock to check the message
--
2.21.0
