linux-ext4 - Re: [BUG] fsck unable to resolve filenames that include '='

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Date:   Wed, 3 Aug 2022 16:31:13 +0200
From:   Lukas Czerner <lczerner@...hat.com>
To:     Daniel Ng <danielng@...gle.com>
Cc:     linux-ext4@...r.kernel.org,
        Sarthak Kukreti <sarthakkukreti@...gle.com>
Subject: Re: [BUG] fsck unable to resolve filenames that include '='

On Tue, Aug 02, 2022 at 06:21:56PM +1000, Daniel Ng wrote:
> Hi,
> 
> I've run into an issue when trying to use fsck with an ext4 image when
> it has '=' in its name.
> 
> Repro steps:
> 1. fallocate -l 1G test=.img
> 2. mkfs.ext4 test=.img
> 3. fsck test=.img
> 
> Response:
> 'fsck.ext4: Unable to resolve '<path>/test=.img'
> 
> Expected:
> fsck to do it's thing.
> 
> Observations:
> Originally I wasn't sure what the source was, I thought that maybe
> mkfs wasn't creating the image appropriately.
> However, I've tried:
> - renaming the image
> - creating a hard-link to the image
> 
> Running fsck on either the renamed image, or the hard-link, works as expected.
> 
> Kernel version: Linux version 4.19.251-13516-ga0bcf8d80077
> Environment: Running on a Chromebook
> 
> Kind regards,
> Daniel

Hi Daniel,

yeah, that's a good catch. The problem is that various e2fsprogs tools
(at least tune2fs and e2fsck) are using blkid_get_devname() to get the
device name without ever checking if we already got the actual existing
device name.

The reason to call blkid_get_devname() at all is to get device in the
form of NAME=value (like for example UUID=uuid, or LABEL=volume-label).
However if we blindly pass in the device (or in this case regular file)
name with an equal sign in it, the blkid_get_devname just returns whatever
it can find by that tag. Which is likely nothing.

Unless of course, you're trying to use e2fsck, or tune2fs on a file with
an actual filename LABEL=volume-label and you have actual file system
with 'volume-label' LABEL ;) That's a problematic behavior and depending
on how we go about fixing it it could be potentialy exploitable...

Maybe something like this:

	1. look for the actual block device first
	2. if none is found call blkid_get_devname()
	3. if that didn't return anything maybe see if have a regular
	   file and work with that
	4. if we still get nothing, then we're "Unable to resolve..."

Thoughts?

-Lukas