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. Signed-off-by: Manfred Spraul <manf...@colorfullife.com> Cc: Waiman Long <long...@redhat.com> Cc: Davidlohr Bueso <d...@stgolabs.net> --- ipc/msg.c | 44 ++++++++++++++++++++++++++++++++++++++------ 1 file changed, 38 insertions(+), 6 deletions(-) diff --git a/ipc/msg.c b/ipc/msg.c index 8dec945fa030..e6b20a7e6341 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); } @@ -238,7 +242,14 @@ 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); - WRITE_ONCE(msr->r_msg, ERR_PTR(res)); + + /* + * The barrier is required to ensure that the refcount increase + * inside wake_q_add() is completed before the state is updated. + * + * The barrier pairs with READ_ONCE()+smp_mb__after_ctrl_dep(). + */ + smp_store_release(&msr->r_msg, ERR_PTR(res)); } } @@ -798,13 +809,17 @@ 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 +1169,11 @@ 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 +1202,21 @@ 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 @@ -1192,7 +1224,7 @@ 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; -- 2.21.0