linux-ext4 - Re: [PATCH] ioctl_getfsmap.2: document the GETFSMAP ioctl

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Date:   Mon, 8 May 2017 20:47:56 +0200
From:   Jann Horn <jannh@...gle.com>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     Michael Kerrisk-manpages <mtk.manpages@...il.com>,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-ext4@...r.kernel.org, Linux API <linux-api@...r.kernel.org>,
        linux-man@...r.kernel.org
Subject: Re: [PATCH] ioctl_getfsmap.2: document the GETFSMAP ioctl

On Mon, May 8, 2017 at 8:41 PM, Darrick J. Wong <darrick.wong@...cle.com> wrote:
> On Mon, May 08, 2017 at 12:17:53AM +0200, Jann Horn wrote:
>> On Sun, May 7, 2017 at 5:58 PM, Darrick J. Wong <darrick.wong@...cle.com> wrote:
>> > Document the new GETFSMAP ioctl that returns the physical layout of a
>> > (disk-based) filesystem.
[...]
>> Also: From a quick glance at the XFS implementation, I don't see any
>> privilege checks. Am I missing something, or does this API permit an
>> unprivileged user to determine the number of physical blocks allocated
>> for any inode, even for inodes the user can't ordinarily see in any
>> way?
>
> Correct.

What's your reasoning for why this doesn't create any new potential
security issues? For example, as far as I can tell, this would permit
an unprivileged user to determine with high probability whether a set
of large files with known sizes is stored anywhere in the filesystem, even
across containers or so.