Re: [PATCH net v4] nfc: ... device_is_registered() is data race-able

From: Lin Ma
Date: Thu Apr 28 2022 - 04:50:10 EST


Hello Greg,

>
> It shouldn't be, if you are using it properly :)
>
> [...]
>
> Yes, you should almost never use that call. Seems the nfc subsystem is
> the most common user of it for some reason :(

Cool, and I believe that the current nfc core code does not use it properly. :(

>
> What state are you trying to track here exactly?
>

Forget about the firmware downloading race that raised by Duoming in this channel,
all the netlink handler code in net/nfc/core.c depends on the device_is_registered
macro.

My idea is to introduce a patch like below:

include/net/nfc/nfc.h | 1 +
net/nfc/core.c | 26 ++++++++++++++------------
2 files changed, 15 insertions(+), 12 deletions(-)

diff --git a/include/net/nfc/nfc.h b/include/net/nfc/nfc.h
index 5dee575fbe86..d84e53802b06 100644
--- a/include/net/nfc/nfc.h
+++ b/include/net/nfc/nfc.h
@@ -168,6 +168,7 @@ struct nfc_dev {
int targets_generation;
struct device dev;
bool dev_up;
+ bool dev_register;
bool fw_download_in_progress;
u8 rf_mode;
bool polling;
diff --git a/net/nfc/core.c b/net/nfc/core.c
index dc7a2404efdf..208e6bb0804e 100644
--- a/net/nfc/core.c
+++ b/net/nfc/core.c
@@ -38,7 +38,7 @@ int nfc_fw_download(struct nfc_dev *dev, const char *firmware_name)

device_lock(&dev->dev);

- if (!device_is_registered(&dev->dev)) {
+ if (!dev->dev_register) {
rc = -ENODEV;
goto error;
}
@@ -94,7 +94,7 @@ int nfc_dev_up(struct nfc_dev *dev)

device_lock(&dev->dev);

- if (!device_is_registered(&dev->dev)) {
+ if (!dev->dev_register) {
rc = -ENODEV;
goto error;
}

[...]

@@ -1134,6 +1134,7 @@ int nfc_register_device(struct nfc_dev *dev)
dev->rfkill = NULL;
}
}
+ dev->dev_register = true;
device_unlock(&dev->dev);

rc = nfc_genl_device_added(dev);
@@ -1162,6 +1163,7 @@ void nfc_unregister_device(struct nfc_dev *dev)
"was removed\n", dev_name(&dev->dev));

device_lock(&dev->dev);
+ dev->dev_register = false;
if (dev->rfkill) {
rfkill_unregister(dev->rfkill);
rfkill_destroy(dev->rfkill);
--
2.35.1

The added dev_register variable can function like the original device_is_registered and does not race-able
because of the protection of device_lock.

I think after such a patch is adopted, the reorder version of patch from Duoming
-> https://lists.openwall.net/netdev/2022/04/25/10
can be used to fix the firmware downloading bug.

Do you agree on this or should we use another macro that is suitable than device_is_registered?

> thanks,
>
> greg k-h

Thanks
Lin