Re: [PATCH] Devcoredump: fix use-after-free issue when releasing devcd device

[Greg KH]

On Thu, Oct 26, 2023 at 10:55:21PM -0700, Yu Wang wrote:
> With sample code as below, it may hit use-after-free issue when
> releasing devcd device.
> 
>     struct my_coredump_state {
>         struct completion dump_done;
>         ...
>     };
> 
>     static void my_coredump_free(void *data)
>     {
>         struct my_coredump_state *dump_state = data;
>         ...
>         complete(&dump_state->dump_done);
>     }
> 
>     static void my_dev_release(struct device *dev)
>     {
>         kfree(dev);
>     }
> 
>     static void my_coredump()
>     {
>         struct my_coredump_state dump_state;
>         struct device *new_device =
>             kzalloc(sizeof(*new_device), GFP_KERNEL);
> 
>         ...
>         new_device->release = my_dev_release;
>         device_initialize(new_device);
>         ...
>         device_add(new_device);
>         ...
>         init_completion(&dump_state.dump_done);
>         dev_coredumpm(new_device, NULL, &dump_state, datalen, GFP_KERNEL,
>                       my_coredump_read, my_coredump_free);
>         wait_for_completion(&dump_state.dump_done);
>         device_del(new_device);
>         put_device(new_device);
>     }
> 
> In devcoredump framework, devcd_dev_release() will be called when
> releasing the devcd device, it will call the free() callback first
> and try to delete the symlink in sysfs directory of the failing device.
> Eventhough it has checked 'devcd->failing_dev->kobj.sd' before that,
> there is no mechanism to ensure it's still available when accessing
> it in kernfs_find_ns(), refer to the diagram as below:
> 
>     Thread A was waiting for 'dump_state.dump_done' at #A-1-2 after
>     calling dev_coredumpm().
>     When thread B calling devcd->free() at #B-2-1, it wakes up
>     thread A from point #A-1-2, which will call device_del() to
>     delete the device.
>     If #B-2-2 comes before #A-3-1, but #B-4 comes after #A-4, it
>     will hit use-after-free issue when trying to access
>     'devcd->failing_dev->kobj.sd'.
> 
>     #A-1-1: dev_coredumpm()
>       #A-1-2: wait_for_completion(&dump_state.dump_done)
>       #A-1-3: device_del()
>         #A-2: kobject_del()
>           #A-3-1: sysfs_remove_dir() --> set kobj->sd=NULL
>           #A-3-2: kernfs_put()
>             #A-4: kmem_cache_free() --> free kobj->sd
> 
>     #B-1: devcd_dev_release()
>       #B-2-1: devcd->free(devcd->data)
>       #B-2-2: check devcd->failing_dev->kobj.sd
>       #B-2-3: sysfs_delete_link()
>         #B-3: kernfs_remove_by_name_ns()
>           #B-4: kernfs_find_ns() --> access devcd->failing_dev->kobj.sd
> 
> To fix this issue, put operations on devcd->failing_dev before
> calling the free() callback in devcd_dev_release().
> 
> Signed-off-by: Yu Wang <quic_yyuwang@xxxxxxxxxxx>
> ---
>  drivers/base/devcoredump.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)

Also, what commit id does this fix?

thanks,

greg k-h