Re: [PATCH] fs: fix use-after-free in __fput() when a chardev is removed but a file is still open

[Al Viro]

On Mon, Nov 25, 2019 at 01:53:42PM +0100, Vladis Dronov wrote:
> In a case when a chardev file (like /dev/ptp0) is open but an underlying
> device is removed, closing this file leads to a use-after-free. This
> reproduces easily in a KVM virtual machine:
> 
> # cat openptp0.c
> int main() { ... fp = fopen("/dev/ptp0", "r"); ... sleep(10); }

> static void __fput(struct file *file)
> {   ...
>     if (file->f_op->release)
>         file->f_op->release(inode, file); <<< cdev is kfree'd here

>     if (unlikely(S_ISCHR(inode->i_mode) && inode->i_cdev != NULL &&
>              !(mode & FMODE_PATH))) {
>         cdev_put(inode->i_cdev); <<< cdev fields are accessed here
> 
> because of:
> 
> __fput()
>   posix_clock_release()
>     kref_put(&clk->kref, delete_clock) <<< the last reference
>       delete_clock()
>         delete_ptp_clock()
>           kfree(ptp) <<< cdev is embedded in ptp
>   cdev_put
>     module_put(p->owner) <<< *p is kfree'd
> 
> The fix is to call cdev_put() before file->f_op->release(). This fix the
> class of bugs when a chardev device is removed when its file is open, for
> example:

And what's to prevent rmmod coming and freeing ->release code right as you
are executing it?