Hi Greg, Alan,
On 1/4/2024 8:26 PM, Alan Stern wrote:
On Thu, Jan 04, 2024 at 02:13:51PM +0100, Greg Kroah-Hartman wrote:
On Thu, Jan 04, 2024 at 06:35:38PM +0530, Udipto Goswami wrote:
Hi Greg,
On 1/4/2024 4:14 PM, Greg Kroah-Hartman wrote:
On Thu, Jan 04, 2024 at 03:56:16PM +0530, Udipto Goswami wrote:
Currently,the function update_port_device_state gets the usb_hub from
udev->parent by calling usb_hub_to_struct_hub.
However, in case the actconfig or the maxchild is 0, the usb_hub would
be NULL and upon further accessing to get port_dev would result in null
pointer dereference.
Is this true for any real (or fake) hardware?
We saw this in our QCOM hardwares where lvstest.c was calling
get_dev_desc_store:
usb_set_device_state+0x128/0x17c
create_lvs_device+0x60/0xf8 [lvstest]
get_dev_desc_store+0x94/0x18c [lvstest]
dev_attr_store+0x30/0x48
I think the part of the test procedure is to first unbind the hub driver
which calls hub_disconnect setting the maxchild = 0.
Are you sure lvstest is correct here?
By the commit description of lvstest driver this seems to be the procedure:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/drivers/usb/misc/lvstest.c?h=v6.6.9&id=ce21bfe603b3401c258c415456c915634998e133
As you can see it mentions unbind is necessary before further steps
carried out. Also, since the test was passing before
update_port_device_state was introduced, wasn't doubting this.
Either way, usb_hub_to_struct_hub() can potentially return NULL not only
for maxchild == 0, but other cases like actconfig == NULL or hdev ==
NULL as well, so it isn't wise to access the hub in subsequent line.
This is what happens when people work behind the hub driver's back. :-(
If you can't find another way to fix the problem, you should at least
change the patch to include a comment before the "if (hub)" test,
explaining why it is necessary. Otherwise somebody in the future will
remove the test, because under normal circumstances hub would never be
NULL here.
Thanks for the review Alan. Sure I'll put a comment here stating the
necessity of the check for clarity in the next version.
I agree under normal conditions this won't fail for example even in this
case we unbinded 2-1. Since 1-1 wasn't unbinded that therefore usb1 has
a maxchild still present.
Thanks,
-Udipto
