Input: sysrq - rework re-inject logic

Internally 'disable' the filter when re-injecting Alt-SysRq instead
of relying on input core to suppress delivery of injected events
to the originating handler.

This allows to revert commit 5fdbe44d033d059cc56c2803e6b4dbd8cb4e5e39
which causes problems with existing userspace programs trying to
loopback the events via evdev.

Reported-by: Kristen Carlson Accardi <kristen@linux.intel.com>
Cc: stable@kernel.org
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

drivers/char/sysrq.c

@@ -571,6 +571,7 @@ struct sysrq_state {
 	unsigned int alt_use;
 	bool active;
 	bool need_reinject;
+	bool reinjecting;
 };
 
 static void sysrq_reinject_alt_sysrq(struct work_struct *work)
@@ -581,6 +582,10 @@ static void sysrq_reinject_alt_sysrq(struct work_struct *work)
 	unsigned int alt_code = sysrq->alt_use;
 
 	if (sysrq->need_reinject) {
+		/* we do not want the assignment to be reordered */
+		sysrq->reinjecting = true;
+		mb();
+
 		/* Simulate press and release of Alt + SysRq */
 		input_inject_event(handle, EV_KEY, alt_code, 1);
 		input_inject_event(handle, EV_KEY, KEY_SYSRQ, 1);
@@ -589,6 +594,9 @@ static void sysrq_reinject_alt_sysrq(struct work_struct *work)
 		input_inject_event(handle, EV_KEY, KEY_SYSRQ, 0);
 		input_inject_event(handle, EV_KEY, alt_code, 0);
 		input_inject_event(handle, EV_SYN, SYN_REPORT, 1);
+
+		mb();
+		sysrq->reinjecting = false;
 	}
 }
 
@@ -599,6 +607,13 @@ static bool sysrq_filter(struct input_handle *handle,
 	bool was_active = sysrq->active;
 	bool suppress;
 
+	/*
+	 * Do not filter anything if we are in the process of re-injecting
+	 * Alt+SysRq combination.
+	 */
+	if (sysrq->reinjecting)
+		return false;
+
 	switch (type) {
 
 	case EV_SYN:
@@ -629,7 +644,7 @@ static bool sysrq_filter(struct input_handle *handle,
 				sysrq->alt_use = sysrq->alt;
 				/*
 				 * If nothing else will be pressed we'll need
-				 * to * re-inject Alt-SysRq keysroke.
+				 * to re-inject Alt-SysRq keysroke.
 				 */
 				sysrq->need_reinject = true;
 			}