Drivers: hv: utils: fix crash when device is removed from host side

The crash is observed when a service is being disabled host side while
userspace daemon is connected to the device:

[   90.244859] general protection fault: 0000 [#1] SMP
...
[   90.800082] Call Trace:
[   90.800082]  [<ffffffff81187008>] __fput+0xc8/0x1f0
[   90.800082]  [<ffffffff8118716e>] ____fput+0xe/0x10
...
[   90.800082]  [<ffffffff81015278>] do_signal+0x28/0x580
[   90.800082]  [<ffffffff81086656>] ? finish_task_switch+0xa6/0x180
[   90.800082]  [<ffffffff81443ebf>] ? __schedule+0x28f/0x870
[   90.800082]  [<ffffffffa01ebbaa>] ? hvt_op_read+0x12a/0x140 [hv_utils]
...

The problem is that hvutil_transport_destroy() which does misc_deregister()
freeing the appropriate device is reachable by two paths: module unload
and from util_remove(). While module unload path is protected by .owner in
struct file_operations util_remove() path is not. Freeing the device while
someone holds an open fd for it is a show stopper.

In general, it is not possible to revoke an fd from all users so the only
way to solve the issue is to defer freeing the hvutil_transport structure.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/hv/hv_utils_transport.c b/drivers/hv/hv_utils_transport.c
index 31c2f8649271fb8e98baa545e9c13fc267923866..ee20b5074238534a9a448b112d8a9098295d0bef 100644 (file)
--- a/drivers/hv/hv_utils_transport.c
+++ b/drivers/hv/hv_utils_transport.c
@@ -155,13 +155,22 @@ static int hvt_op_open(struct inode *inode, struct file *file)
        return ret;
 }
 
+static void hvt_transport_free(struct hvutil_transport *hvt)
+{
+       misc_deregister(&hvt->mdev);
+       kfree(hvt->outmsg);
+       kfree(hvt);
+}
+
 static int hvt_op_release(struct inode *inode, struct file *file)
 {
        struct hvutil_transport *hvt;
+       int mode_old;
 
        hvt = container_of(file->f_op, struct hvutil_transport, fops);
 
        mutex_lock(&hvt->lock);
+       mode_old = hvt->mode;
        if (hvt->mode != HVUTIL_TRANSPORT_DESTROY)
                hvt->mode = HVUTIL_TRANSPORT_INIT;
        /*
@@ -171,6 +180,9 @@ static int hvt_op_release(struct inode *inode, struct file *file)
        hvt_reset(hvt);
        mutex_unlock(&hvt->lock);
 
+       if (mode_old == HVUTIL_TRANSPORT_DESTROY)
+               hvt_transport_free(hvt);
+
        return 0;
 }
 
@@ -304,17 +316,25 @@ err_free_hvt:
 
 void hvutil_transport_destroy(struct hvutil_transport *hvt)
 {
+       int mode_old;
+
        mutex_lock(&hvt->lock);
+       mode_old = hvt->mode;
        hvt->mode = HVUTIL_TRANSPORT_DESTROY;
        wake_up_interruptible(&hvt->outmsg_q);
        mutex_unlock(&hvt->lock);
 
+       /*
+        * In case we were in 'chardev' mode we still have an open fd so we
+        * have to defer freeing the device. Netlink interface can be freed
+        * now.
+        */
        spin_lock(&hvt_list_lock);
        list_del(&hvt->list);
        spin_unlock(&hvt_list_lock);
        if (hvt->cn_id.idx > 0 && hvt->cn_id.val > 0)
                cn_del_callback(&hvt->cn_id);
-       misc_deregister(&hvt->mdev);
-       kfree(hvt->outmsg);
-       kfree(hvt);
+
+       if (mode_old != HVUTIL_TRANSPORT_CHARDEV)
+               hvt_transport_free(hvt);
 }