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authorManfred Spraul <manfred@colorfullife.com>2020-02-03 17:34:36 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2020-02-04 03:05:23 +0000
commitc5b2cbdbdac563f46ecd5e187253ab1abbd6fc04 (patch)
tree43ccc55872802a1209dc849d3c9931ae228df31a /ipc
parented29f171518cbe11c81e8c20d393bb094a9e2ce7 (diff)
ipc/mqueue.c: update/document memory barriers
Update and document memory barriers for mqueue.c: - ewp->state is read without any locks, thus READ_ONCE is required. - add smp_aquire__after_ctrl_dep() after the READ_ONCE, we need acquire semantics if the value is STATE_READY. - use wake_q_add_safe() - document why __set_current_state() may be used: Reading task->state cannot happen before the wake_q_add() call, which happens while holding info->lock. Thus the spin_unlock() is the RELEASE, and the spin_lock() is the ACQUIRE. For completeness: there is also a 3 CPU scenario, if the to be woken up task is already on another wake_q. Then: - CPU1: spin_unlock() of the task that goes to sleep is the RELEASE - CPU2: the spin_lock() of the waker is the ACQUIRE - CPU2: smp_mb__before_atomic inside wake_q_add() is the RELEASE - CPU3: smp_mb__after_spinlock() inside try_to_wake_up() is the ACQUIRE Link: http://lkml.kernel.org/r/20191020123305.14715-4-manfred@colorfullife.com Signed-off-by: Manfred Spraul <manfred@colorfullife.com> Reviewed-by: Davidlohr Bueso <dbueso@suse.de> Cc: Waiman Long <longman@redhat.com> 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>
Diffstat (limited to 'ipc')
-rw-r--r--ipc/mqueue.c92
1 files changed, 78 insertions, 14 deletions
diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index 270456530f6a..49a05ba3000d 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -63,6 +63,66 @@ struct posix_msg_tree_node {
int priority;
};
+/*
+ * Locking:
+ *
+ * Accesses to a message queue are synchronized by acquiring info->lock.
+ *
+ * There are two notable exceptions:
+ * - The actual wakeup of a sleeping task is performed using the wake_q
+ * framework. info->lock is already released when wake_up_q is called.
+ * - The exit codepaths after sleeping check ext_wait_queue->state without
+ * any locks. If it is STATE_READY, then the syscall is completed without
+ * acquiring info->lock.
+ *
+ * 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.
+ *
+ * This prevents the following races:
+ *
+ * 1) With the simple wake_q_add(), the task could be gone already before
+ * the increase of the reference happens
+ * Thread A
+ * Thread B
+ * WRITE_ONCE(wait.state, STATE_NONE);
+ * schedule_hrtimeout()
+ * wake_q_add(A)
+ * if (cmpxchg()) // success
+ * ->state = STATE_READY (reordered)
+ * <timeout returns>
+ * if (wait.state == STATE_READY) return;
+ * sysret to user space
+ * 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) Without proper _release/_acquire barriers, the woken up task
+ * could read stale data
+ *
+ * Thread A
+ * Thread B
+ * do_mq_timedreceive
+ * WRITE_ONCE(wait.state, STATE_NONE);
+ * schedule_hrtimeout()
+ * state = STATE_READY;
+ * <timeout returns>
+ * if (wait.state == STATE_READY) return;
+ * msg_ptr = wait.msg; // Access to stale data!
+ * receiver->msg = message; (reordered)
+ *
+ * Solution: use _release and _acquire barriers.
+ *
+ * 3) 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
+ * acquire memory barrier.
+ */
+
struct ext_wait_queue { /* queue of sleeping tasks */
struct task_struct *task;
struct list_head list;
@@ -646,18 +706,23 @@ static int wq_sleep(struct mqueue_inode_info *info, int sr,
wq_add(info, sr, ewp);
for (;;) {
+ /* memory barrier not required, we hold info->lock */
__set_current_state(TASK_INTERRUPTIBLE);
spin_unlock(&info->lock);
time = schedule_hrtimeout_range_clock(timeout, 0,
HRTIMER_MODE_ABS, CLOCK_REALTIME);
- if (ewp->state == STATE_READY) {
+ if (READ_ONCE(ewp->state) == STATE_READY) {
+ /* see MQ_BARRIER for purpose/pairing */
+ smp_acquire__after_ctrl_dep();
retval = 0;
goto out;
}
spin_lock(&info->lock);
- if (ewp->state == STATE_READY) {
+
+ /* we hold info->lock, so no memory barrier required */
+ if (READ_ONCE(ewp->state) == STATE_READY) {
retval = 0;
goto out_unlock;
}
@@ -923,16 +988,11 @@ static inline void __pipelined_op(struct wake_q_head *wake_q,
struct ext_wait_queue *this)
{
list_del(&this->list);
- wake_q_add(wake_q, this->task);
- /*
- * Rely on the implicit cmpxchg barrier from wake_q_add such
- * that we can ensure that updating receiver->state is the last
- * write operation: As once set, the receiver can continue,
- * and if we don't have the reference count from the wake_q,
- * yet, at that point we can later have a use-after-free
- * condition and bogus wakeup.
- */
- this->state = STATE_READY;
+ get_task_struct(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
@@ -1049,7 +1109,9 @@ static int do_mq_timedsend(mqd_t mqdes, const char __user *u_msg_ptr,
} else {
wait.task = current;
wait.msg = (void *) msg_ptr;
- wait.state = STATE_NONE;
+
+ /* memory barrier not required, we hold info->lock */
+ WRITE_ONCE(wait.state, STATE_NONE);
ret = wq_sleep(info, SEND, timeout, &wait);
/*
* wq_sleep must be called with info->lock held, and
@@ -1152,7 +1214,9 @@ static int do_mq_timedreceive(mqd_t mqdes, char __user *u_msg_ptr,
ret = -EAGAIN;
} else {
wait.task = current;
- wait.state = STATE_NONE;
+
+ /* memory barrier not required, we hold info->lock */
+ WRITE_ONCE(wait.state, STATE_NONE);
ret = wq_sleep(info, RECV, timeout, &wait);
msg_ptr = wait.msg;
}