The check if the queue is full and adding current to the wait queue of pending
msgsnd() operations (ss_add()) must be atomic.
Otherwise:
- the thread that performs msgsnd() finds a full queue and decides to sleep.
- the thread that performs msgrcv() calls first reads all messages from the
queue and then sleep, because the queue is empty.
- the msgrcv() calls do not perform any wakeups, because the msgsnd() task
has not yet called ss_add().
- then the msgsnd()-thread first calls ss_add() and then sleeps.
Net result: msgsnd() and msgrcv() both sleep forever.
Observed with msgctl08 from ltp with a preemptible kernel.
Fix: Call ipc_lock_object() before performing the check.
The patch also moves security_msg_queue_msgsnd() under ipc_lock_object:
- msgctl(IPC_SET) explicitely mentions that it tries to expunge any pending
operations that are not allowed anymore with the new permissions.
If security_msg_queue_msgsnd() is called without locks, then there might be
races.
- it makes the patch much simpler.
Reported-by: Vineet Gupta <Vineet.Gupta1@xxxxxxxxxxxx>
Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
---
ipc/msg.c | 12 +++++-------
1 file changed, 5 insertions(+), 7 deletions(-)
diff --git a/ipc/msg.c b/ipc/msg.c
index 9f29d9e..b65fdf1 100644
--- a/ipc/msg.c
+++ b/ipc/msg.c
@@ -680,16 +680,18 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
goto out_unlock1;
}
+ ipc_lock_object(&msq->q_perm);
+
for (;;) {
struct msg_sender s;
err = -EACCES;
if (ipcperms(ns, &msq->q_perm, S_IWUGO))
- goto out_unlock1;
+ goto out_unlock0;
err = security_msg_queue_msgsnd(msq, msg, msgflg);
if (err)
- goto out_unlock1;
+ goto out_unlock0;
if (msgsz + msq->q_cbytes <= msq->q_qbytes &&
1 + msq->q_qnum <= msq->q_qbytes) {
@@ -699,10 +701,9 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
/* queue full, wait: */
if (msgflg & IPC_NOWAIT) {
err = -EAGAIN;
- goto out_unlock1;
+ goto out_unlock0;
}
- ipc_lock_object(&msq->q_perm);
ss_add(msq, &s);
if (!ipc_rcu_getref(msq)) {
@@ -730,10 +731,7 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
goto out_unlock0;
}
- ipc_unlock_object(&msq->q_perm);
}
-
- ipc_lock_object(&msq->q_perm);
msq->q_lspid = task_tgid_vnr(current);
msq->q_stime = get_seconds();
--
1.8.3.1
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