Re: [PATCH] uio: ensure class is registered before devices

[Greg Kroah-Hartman]

On Fri, Jun 15, 2018 at 05:52:49PM +0200, Alexandre Belloni wrote:
> When both uio and the uio drivers are built in the kernel, it is possible
> for a driver to register devices before the uio class is registered.
> 
> This may result in a NULL pointer dereference later on in
> get_device_parent() when accessing the class glue_dirs spinlock.
> 
> The trace looks like that:
> 
> Unable to handle kernel NULL pointer dereference at virtual address 00000140
> [...]
> Process swapper/0 (pid: 1, stack limit = 0xffff000008070000)
> Call trace:
> Exception stack(0xffff000008073810 to 0xffff000008073950)
> 3800:                                   0000000000000140 0000000000000001
> 3820: ffff800078f60000 ffffffffffffffff 0000000000000000 0000000000000000
> 3840: 0000000000000d39 07650764075f0774 0765076307690776 077207610770075f
> 3860: 07200774076e0765 0720072007200720 0720072007200720 0720072007200720
> 3880: 0720072007200720 ffffffffffffffff 0000000000000000 ffff0000089e88c0
> 38a0: 0000000000000010 ffff000008e0c000 ffff8000780990b0 ffff8000780f4ec0
> 38c0: ffff000008c41a48 ffff000008c41a30 ffff000008a59660 ffff000008c41a30
> 38e0: ffff000008a59660 ffff8000780f4c80 ffff000008c41000 ffff000008073950
> 3900: ffff0000084f3bb0 ffff000008073950 ffff0000089cc234 0000000040000045
> 3920: 00000000000005e7 ffff000008a59660 ffffffffffffffff 0000000000000000
> 3940: ffff000008073950 ffff0000089cc234
> [<ffff0000089cc234>] _raw_spin_lock+0x14/0x48
> [<ffff0000084f56bc>] device_add+0x154/0x6a0
> [<ffff0000084f5e48>] device_create_groups_vargs+0x120/0x128
> [<ffff0000084f5edc>] device_create+0x54/0x60
> [<ffff0000086e72c0>] __uio_register_device+0x120/0x4a8
> [<ffff000008528b7c>] jaguar2_pci_probe+0x2d4/0x558
> [<ffff0000083fc18c>] local_pci_probe+0x3c/0xb8
> [<ffff0000083fd81c>] pci_device_probe+0x11c/0x180
> [<ffff0000084f88bc>] driver_probe_device+0x22c/0x2d8
> [<ffff0000084f8a24>] __driver_attach+0xbc/0xc0
> [<ffff0000084f69fc>] bus_for_each_dev+0x4c/0x98
> [<ffff0000084f81b8>] driver_attach+0x20/0x28
> [<ffff0000084f7d08>] bus_add_driver+0x1b8/0x228
> [<ffff0000084f93c0>] driver_register+0x60/0xf8
> [<ffff0000083fb918>] __pci_register_driver+0x40/0x48
> 
> Return EPROBE_DEFER in that case so the driver can register the device
> later.
> 
> Signed-off-by: Alexandre Belloni <alexandre.belloni@xxxxxxxxxxx>
> ---
> 
> Hi Greg,
> 
> I'm not sure using struct class::p to test for class registration is really
> something we should do but this allows to have a small patch.
> 
> The other solutions to fix this are:
>  - use a similar change but with a boolean to store whether the class has been
>    registered.
>  - or instead of EPROBE_DEFER, just register the class when the first uio
>    device is registered.
>  _ or stop allowing to compile uio as a module and use subsys_initcall instead
>  of module_init
> 
> What do you think?
> 
>  drivers/uio/uio.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/uio/uio.c b/drivers/uio/uio.c
> index e8f4ac9400ea..3fb84ab3dd99 100644
> --- a/drivers/uio/uio.c
> +++ b/drivers/uio/uio.c
> @@ -853,6 +853,9 @@ int __uio_register_device(struct module *owner,
>  	struct uio_device *idev;
>  	int ret = 0;
>  
> +	if (!uio_class.p)
> +		return -EPROBE_DEFER;

Ick, you are right, don't touch the .p field please.  We just need a "is
uio all initialized" flag for the class here that we should check
instead.

thanks,

greg k-h