linux-kernel - Re: [PATCH] loop: prevent bdev freeing while device in use

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Date:	Mon, 1 Apr 2013 10:00:35 -0700
From:	Anatol Pomozov <anatol.pomozov@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Theodore Ts'o" <tytso@....edu>, Salman Qazi <sqazi@...gle.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Guo Chao <yan@...ux.vnet.ibm.com>
Subject: Re: [PATCH] loop: prevent bdev freeing while device in use

Hi

On Mon, Apr 1, 2013 at 8:16 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Mon, Apr 1, 2013 at 4:58 AM, Anatol Pomozov <anatol.pomozov@...il.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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