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SubjectRe: [PATCH] loop: prevent bdev freeing while device in use
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Hi

On Mon, Apr 1, 2013 at 8:16 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Mon, Apr 1, 2013 at 4:58 AM, Anatol Pomozov <anatol.pomozov@gmail.com> wrote:
>>
>> To prevent use-after-free we need to hold device inode in loop_set_fd()
>> and put it later in loop_clr_fd().
>
> Is there something that guarantees that there's only one loop_set_fd()
> and one paired loop_clr_fd()?

Yes there is such guarantee.

Every time we call loop_set_fd() we check that loop_device->lo_state
is Lo_unbound and set it to Lo_bound If somebody will try to set_fd
again it will get EBUSY. And if we try to loop_clr_fd() on unbound
loop device we'll get ENXIO.

loop_set_fd/loop_clr_fd (and any other loop ioctl) is called under
loop_device->lo_ctl_mutex.

>
> IOW, what protects us from somebody doing loop_clr_fd() on a device
> that hasn't been set up yet, or doing multiple loop_set_fd calls?

> I suspect the "lo->lo_state" is part of the answer, but it's very much
> not obvious, and I'd want this to be explicitly explained.
>
> Linus


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