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Subject[PATCH v2 4/4] Drivers: hv: utils: fix crash when device is removed from host side
Date
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>
---
drivers/hv/hv_utils_transport.c | 26 +++++++++++++++++++++++---
1 file changed, 23 insertions(+), 3 deletions(-)

diff --git a/drivers/hv/hv_utils_transport.c b/drivers/hv/hv_utils_transport.c
index 31c2f86..ee20b50 100644
--- 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);
}
--
2.4.3


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