Input: uinput - avoid crash when sending FF request to device going away

If FF request comes in while uinput device is going away,
uinput_request_send() will fail with -ENODEV, and uinput_request_submit()
will attempt to mark the slot as unused by calling uinput_request_done().
Unfortunately in this case we haven't initialized request->done completion
yet, and we get a crash:

[   39.402036] BUG: spinlock bad magic on CPU#1, fftest/3108
[   39.402046]  lock: 0xffff88006a93bb00, .magic: 00000000, .owner: /39, .owner_cpu: 1217155072
[   39.402055] CPU: 1 PID: 3108 Comm: fftest Tainted: G        W 4.13.0+ #15
[   39.402059] Hardware name: LENOVO 20HQS0EG02/20HQS0EG02, BIOS N1MET37W (1.22 ) 07/04/2017
[   39.402064]  0000000000000086 f0fad82f3ceaa120 ffff88006a93b9a0 ffffffff9de941bb
[   39.402077]  ffff88026df8ae00 ffff88006a93bb00 ffff88006a93b9c0 ffffffff9dca62b7
[   39.402088]  ffff88006a93bb00 ffff88006a93baf8 ffff88006a93b9e0 ffffffff9dca62e7
[   39.402099] Call Trace:
[   39.402112]  [<ffffffff9de941bb>] dump_stack+0x4d/0x63
[   39.402123]  [<ffffffff9dca62b7>] spin_dump+0x97/0x9c
[   39.402130]  [<ffffffff9dca62e7>] spin_bug+0x2b/0x2d
[   39.402138]  [<ffffffff9dca6373>] do_raw_spin_lock+0x28/0xfd
[   39.402147]  [<ffffffff9e3055cd>] _raw_spin_lock_irqsave+0x19/0x1f
[   39.402154]  [<ffffffff9dca05b7>] complete+0x1d/0x48
[   39.402162]  [<ffffffffc04f30af>] 0xffffffffc04f30af
[   39.402167]  [<ffffffffc04f468c>] 0xffffffffc04f468c
[   39.402177]  [<ffffffff9dd59c16>] ? __slab_free+0x22f/0x359
[   39.402184]  [<ffffffff9dcc13e9>] ? tk_clock_read+0xc/0xe
[   39.402189]  [<ffffffffc04f471f>] 0xffffffffc04f471f
[   39.402195]  [<ffffffff9dc9ffe5>] ? __wake_up+0x44/0x4b
[   39.402200]  [<ffffffffc04f3240>] ? 0xffffffffc04f3240
[   39.402207]  [<ffffffff9e0f57f3>] erase_effect+0xa1/0xd2
[   39.402214]  [<ffffffff9e0f58c6>] input_ff_flush+0x43/0x5c
[   39.402219]  [<ffffffffc04f32ad>] 0xffffffffc04f32ad
[   39.402227]  [<ffffffff9e0f174f>] input_flush_device+0x3d/0x51
[   39.402234]  [<ffffffff9e0f69ae>] evdev_flush+0x49/0x5c
[   39.402243]  [<ffffffff9dd62d6e>] filp_close+0x3f/0x65
[   39.402253]  [<ffffffff9dd7dcf7>] put_files_struct+0x66/0xc1
[   39.402261]  [<ffffffff9dd7ddeb>] exit_files+0x47/0x4e
[   39.402270]  [<ffffffff9dc6b329>] do_exit+0x483/0x969
[   39.402278]  [<ffffffff9dc73211>] ? recalc_sigpending_tsk+0x3d/0x44
[   39.402285]  [<ffffffff9dc6c7a2>] do_group_exit+0x42/0xb0
[   39.402293]  [<ffffffff9dc767e1>] get_signal+0x58d/0x5bf
[   39.402300]  [<ffffffff9dc03701>] do_signal+0x37/0x53e
[   39.402307]  [<ffffffff9e0f8401>] ? evdev_ioctl_handler+0xac8/0xb04
[   39.402314]  [<ffffffff9e0f8464>] ? evdev_ioctl+0x10/0x12
[   39.402321]  [<ffffffff9dd74cfa>] ? do_vfs_ioctl+0x42e/0x501
[   39.402328]  [<ffffffff9dc0170e>] prepare_exit_to_usermode+0x66/0x90
[   39.402333]  [<ffffffff9dc0181b>] syscall_return_slowpath+0xe3/0xec
[   39.402339]  [<ffffffff9e305b7b>] int_ret_from_sys_call+0x25/0x8f

While we could solve this by simply initializing the completion earlier, we
are better off rearranging the code a bit so we avoid calling complete() on
requests that we did not send out. This patch consolidates marking request
slots as free in one place (in uinput_request_submit(), the same place
where we acquire them) and having everyone else simply signal completion
of the requests.

Fixes: 00ce756ce53a ("Input: uinput - mark failed submission requests as free")
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/misc/uinput.c

@@ -98,14 +98,15 @@ static int uinput_request_reserve_slot(struct uinput_device *udev,
 					uinput_request_alloc_id(udev, request));
 }
 
-static void uinput_request_done(struct uinput_device *udev,
-				struct uinput_request *request)
+static void uinput_request_release_slot(struct uinput_device *udev,
+					unsigned int id)
 {
 	/* Mark slot as available */
-	udev->requests[request->id] = NULL;
-	wake_up(&udev->requests_waitq);
+	spin_lock(&udev->requests_lock);
+	udev->requests[id] = NULL;
+	spin_unlock(&udev->requests_lock);
 
-	complete(&request->done);
+	wake_up(&udev->requests_waitq);
 }
 
 static int uinput_request_send(struct uinput_device *udev,
@@ -138,20 +139,22 @@ static int uinput_request_send(struct uinput_device *udev,
 static int uinput_request_submit(struct uinput_device *udev,
 				 struct uinput_request *request)
 {
-	int error;
+	int retval;
 
-	error = uinput_request_reserve_slot(udev, request);
-	if (error)
-		return error;
+	retval = uinput_request_reserve_slot(udev, request);
+	if (retval)
+		return retval;
 
-	error = uinput_request_send(udev, request);
-	if (error) {
-		uinput_request_done(udev, request);
-		return error;
-	}
+	retval = uinput_request_send(udev, request);
+	if (retval)
+		goto out;
 
 	wait_for_completion(&request->done);
-	return request->retval;
+	retval = request->retval;
+
+ out:
+	uinput_request_release_slot(udev, request->id);
+	return retval;
 }
 
 /*
@@ -169,7 +172,7 @@ static void uinput_flush_requests(struct uinput_device *udev)
 		request = udev->requests[i];
 		if (request) {
 			request->retval = -ENODEV;
-			uinput_request_done(udev, request);
+			complete(&request->done);
 		}
 	}
 
@@ -957,7 +960,7 @@ static long uinput_ioctl_handler(struct file *file, unsigned int cmd,
 			}
 
 			req->retval = ff_up.retval;
-			uinput_request_done(udev, req);
+			complete(&req->done);
 			goto out;
 
 		case UI_END_FF_ERASE:
@@ -973,7 +976,7 @@ static long uinput_ioctl_handler(struct file *file, unsigned int cmd,
 			}
 
 			req->retval = ff_erase.retval;
-			uinput_request_done(udev, req);
+			complete(&req->done);
 			goto out;
 	}