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Subject[PATCH] firewire: cdev: prevent race between first get_info ioctl and bus reset event queuing
Between open(2) of a /dev/fw* and the first FW_CDEV_IOC_GET_INFO
ioctl(2) on it, the kernel already queues FW_CDEV_EVENT_BUS_RESET events
to be read(2) by the client. The get_info ioctl is practically always
issued right away after open, hence this condition only occurs if the
client opens during a bus reset, especially during a rapid series of bus
resets.

The problem with this condition is two-fold:

- These bus reset events carry the (as yet undocumented) @closure
value of 0. But it is not the kernel's place to choose closures;
they are privat to the client. E.g., this 0 value forced from the
kernel makes it unsafe for clients to dereference it as a pointer to
a closure object without NULL pointer check.

- It is impossible for clients to determine the relative order of bus
reset events from get_info ioctl(2) versus those from read(2),
except in one way: By comparison of closure values. Again, such a
procedure imposes complexity on clients and reduces freedom in use
of the bus reset closure.

So, change the ABI to suppress queuing of bus reset events before the
first FW_CDEV_IOC_GET_INFO ioctl was issued by the client.

Note, this ABI change cannot be version-controlled. The kernel cannot
distinguish old from new clients before the first FW_CDEV_IOC_GET_INFO
ioctl.

We will try to back-merge this change into currently maintained stable/
longterm series, and we only document the new behaviour. The old
behavior is now considered a kernel bug, which it basically is.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Cc: <stable@kernel.org>
---
drivers/firewire/core-cdev.c | 18 ++++++++++--------
include/linux/firewire-cdev.h | 3 +++
2 files changed, 13 insertions(+), 8 deletions(-)
Index: b/drivers/firewire/core-cdev.c
===================================================================
--- a/drivers/firewire/core-cdev.c
+++ b/drivers/firewire/core-cdev.c
@@ -253,14 +253,11 @@ static int fw_device_op_open(struct inod
init_waitqueue_head(&client->wait);
init_waitqueue_head(&client->tx_flush_wait);
INIT_LIST_HEAD(&client->phy_receiver_link);
+ INIT_LIST_HEAD(&client->link);
kref_init(&client->kref);

file->private_data = client;

- mutex_lock(&device->client_list_mutex);
- list_add_tail(&client->link, &device->client_list);
- mutex_unlock(&device->client_list_mutex);
-
return nonseekable_open(inode, file);
}

@@ -451,15 +448,20 @@ static int ioctl_get_info(struct client
if (ret != 0)
return -EFAULT;

+ mutex_lock(&client->device->client_list_mutex);
+
client->bus_reset_closure = a->bus_reset_closure;
if (a->bus_reset != 0) {
fill_bus_reset_event(&bus_reset, client);
- if (copy_to_user(u64_to_uptr(a->bus_reset),
- &bus_reset, sizeof(bus_reset)))
- return -EFAULT;
+ ret = copy_to_user(u64_to_uptr(a->bus_reset),
+ &bus_reset, sizeof(bus_reset));
}
+ if (ret == 0 && list_empty(&client->link))
+ list_add_tail(&client->link, &client->device->client_list);

- return 0;
+ mutex_unlock(&client->device->client_list_mutex);
+
+ return ret ? -EFAULT : 0;
}

static int add_client_resource(struct client *client,
Index: b/include/linux/firewire-cdev.h
===================================================================
--- a/include/linux/firewire-cdev.h
+++ b/include/linux/firewire-cdev.h
@@ -475,6 +475,9 @@ union fw_cdev_event {
* of the bus. This does not cause a bus reset to happen.
* @bus_reset_closure: Value of &closure in this and subsequent bus reset events
* @card: The index of the card this device belongs to
+ *
+ * As a side effect, reception of %FW_CDEV_EVENT_BUS_RESET events to be read(2)
+ * is started by this ioctl.
*/
struct fw_cdev_get_info {
__u32 version;

--
Stefan Richter
-=====-==-== -=== -=--=
http://arcgraph.de/sr/

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