ipc/msg.c: update and document memory barriers

Transfer findings from ipc/mqueue.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.

- use smp_store_release().  In theory, the refount may have been
  decreased to 0 already when wake_q_add() tries to get a reference.

Link: http://lkml.kernel.org/r/20191020123305.14715-5-manfred@colorfullife.com
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Waiman Long <longman@redhat.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: <1vier1@web.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/ipc/msg.c b/ipc/msg.c
index 8dec945..82ca36b 100644 (file)
--- a/ipc/msg.c
+++ b/ipc/msg.c
@@ -61,6 +61,16 @@ struct msg_queue {
        struct list_head q_senders;
 } __randomize_layout;
 
+/*
+ * MSG_BARRIER Locking:
+ *
+ * Similar to the optimization used in ipc/mqueue.c, one syscall return path
+ * does not acquire any locks when it sees that a message exists in
+ * msg_receiver.r_msg. Therefore r_msg is set using smp_store_release()
+ * and accessed using READ_ONCE()+smp_acquire__after_ctrl_dep(). In addition,
+ * wake_q_add_safe() is used. See ipc/mqueue.c for more details
+ */
+
 /* one msg_receiver structure for each sleeping receiver */
 struct msg_receiver {
        struct list_head        r_list;
@@ -184,6 +194,10 @@ 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);
 }
@@ -237,8 +251,11 @@ static void expunge_all(struct msg_queue *msq, int res,
        struct msg_receiver *msr, *t;
 
        list_for_each_entry_safe(msr, t, &msq->q_receivers, r_list) {
-               wake_q_add(wake_q, msr->r_tsk);
-               WRITE_ONCE(msr->r_msg, ERR_PTR(res));
+               get_task_struct(msr->r_tsk);
+
+               /* see MSG_BARRIER for purpose/pairing */
+               smp_store_release(&msr->r_msg, ERR_PTR(res));
+               wake_q_add_safe(wake_q, msr->r_tsk);
        }
 }
 
@@ -798,13 +815,17 @@ 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);
-                               WRITE_ONCE(msr->r_msg, ERR_PTR(-E2BIG));
+
+                               /* See expunge_all regarding memory barrier */
+                               smp_store_release(&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);
-                               WRITE_ONCE(msr->r_msg, msg);
+
+                               /* See expunge_all regarding memory barrier */
+                               smp_store_release(&msr->r_msg, msg);
                                return 1;
                        }
                }
@@ -1154,7 +1175,11 @@ static long do_msgrcv(int msqid, void __user *buf, size_t bufsz, long msgtyp, in
                        msr_d.r_maxsize = INT_MAX;
                else
                        msr_d.r_maxsize = bufsz;
-               msr_d.r_msg = ERR_PTR(-EAGAIN);
+
+               /* memory barrier not require due to ipc_lock_object() */
+               WRITE_ONCE(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 +1208,12 @@ 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)) {
+                       /* see MSG_BARRIER for purpose/pairing */
+                       smp_acquire__after_ctrl_dep();
+
                        goto out_unlock1;
+               }
 
                 /*
                  * ... or see -EAGAIN, acquire the lock to check the message
@@ -1192,7 +1221,7 @@ static long do_msgrcv(int msqid, void __user *buf, size_t bufsz, long msgtyp, in
                  */
                ipc_lock_object(&msq->q_perm);
 
-               msg = msr_d.r_msg;
+               msg = READ_ONCE(msr_d.r_msg);
                if (msg != ERR_PTR(-EAGAIN))
                        goto out_unlock0;