linux-kernel - Re: overlayfs access checks on underlying layers

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Message-ID: <CAJfpegv6y56k1-Tewu-Pu3x1W75LL6qYB6Hb6=n+2BJoNfEigA@mail.gmail.com>
Date:   Thu, 29 Nov 2018 20:47:22 +0100
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Stephen Smalley <sds@...ho.nsa.gov>
Cc:     Vivek Goyal <vgoyal@...hat.com>,
        Ondrej Mosnacek <omosnace@...hat.com>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Mark Salyzyn <salyzyn@...roid.com>,
        Paul Moore <paul@...l-moore.com>, linux-kernel@...r.kernel.org,
        overlayfs <linux-unionfs@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, selinux@...r.kernel.org,
        Daniel J Walsh <dwalsh@...hat.com>
Subject: Re: overlayfs access checks on underlying layers

On Thu, Nov 29, 2018 at 5:14 PM Stephen Smalley <sds@...ho.nsa.gov> wrote:

> Possibly I misunderstood you, but I don't think we want to copy-up on
> permission denial, as that would still allow the mounter to read/write
> special files or execute regular files to which it would normally be
> denied access, because the copy would inherit the context specified by
> the mounter in the context mount case.  It still represents an
> escalation of privilege for the mounter.  In contrast, the copy-up on
> write behavior does not allow the mounter to do anything it could not do
> already (i.e. read from the lower, write to the upper).

Let's get this straight:  when file is copied up, it inherits label
from context=, not from label of lower file?

Next question: permission to change metadata is tied to permission to
open?  Is it possible that open is denied, but metadata can be
changed?

DAC model allows this: metadata change is tied to ownership, not mode
bits.   And different capability flag.

If the same is true for MAC, then the pre-v4.20-rc1 is already
susceptible to the privilege escalation you describe, right?

Thanks,
Miklos