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Message-ID: <1534974222.2902.15.camel@codethink.co.uk>
Date: Wed, 22 Aug 2018 22:43:42 +0100
From: Ben Hutchings <ben.hutchings@...ethink.co.uk>
To: Theodore Tso <tytso@....edu>, Jens Axboe <axboe@...nel.dk>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
LKML <linux-kernel@...r.kernel.org>, stable@...r.kernel.org
Subject: Re: [PATCH 4.4 40/43] loop: add recursion validation to
LOOP_CHANGE_FD
On Mon, 2018-07-16 at 09:36 +0200, Greg Kroah-Hartman wrote:
> 4.4-stable review patch. If anyone has any objections, please let me know.
>
> ------------------
>
> From: Theodore Ts'o <tytso@....edu>
>
> commit d2ac838e4cd7e5e9891ecc094d626734b0245c99 upstream.
>
> Refactor the validation code used in LOOP_SET_FD so it is also used in
> LOOP_CHANGE_FD. Otherwise it is possible to construct a set of loop
> devices that all refer to each other. This can lead to a infinite
> loop in starting with "while (is_loop_device(f)) .." in loop_set_fd().
>
> Fix this by refactoring out the validation code and using it for
> LOOP_CHANGE_FD as well as LOOP_SET_FD.
[...]
> +static int loop_validate_file(struct file *file, struct block_device *bdev)
> +{
> + struct inode *inode = file->f_mapping->host;
> + struct file *f = file;
> +
> + /* Avoid recursion */
> + while (is_loop_device(f)) {
> + struct loop_device *l;
> +
> + if (f->f_mapping->host->i_bdev == bdev)
> + return -EBADF;
> +
> + l = f->f_mapping->host->i_bdev->bd_disk->private_data;
> + if (l->lo_state == Lo_unbound) {
> + return -EINVAL;
> + }
> + f = l->lo_backing_file;
This looks racy; I don't see anything that prevents a lower loop device
from being reconfigured while this walks down the device stack.
(But this isn't a new problem.)
Ben.
> + }
> + if (!S_ISREG(inode->i_mode) && !S_ISBLK(inode->i_mode))
> + return -EINVAL;
> + return 0;
> +}
[...]
--
Ben Hutchings, Software Developer Codethink Ltd
https://www.codethink.co.uk/ Dale House, 35 Dale Street
Manchester, M1 2HF, United Kingdom
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