From: Vivek Goyal <vgoyal@redhat.com>
To: Stephen Smalley <sds@tycho.nsa.gov>
Cc: Miklos Szeredi <miklos@szeredi.hu>,
Ondrej Mosnacek <omosnace@redhat.com>,
"J. Bruce Fields" <bfields@fieldses.org>,
Mark Salyzyn <salyzyn@android.com>,
Paul Moore <paul@paul-moore.com>,
linux-kernel@vger.kernel.org,
overlayfs <linux-unionfs@vger.kernel.org>,
linux-fsdevel@vger.kernel.org, selinux@vger.kernel.org,
Daniel J Walsh <dwalsh@redhat.com>
Subject: Re: overlayfs access checks on underlying layers
Date: Tue, 4 Dec 2018 10:22:48 -0500 [thread overview]
Message-ID: <20181204152248.GB21509@redhat.com> (raw)
In-Reply-To: <20181204151549.GA21509@redhat.com>
On Tue, Dec 04, 2018 at 10:15:49AM -0500, Vivek Goyal wrote:
> On Tue, Dec 04, 2018 at 09:30:53AM -0500, Stephen Smalley wrote:
> > On 12/4/18 8:32 AM, Miklos Szeredi wrote:
> > > On Thu, Nov 29, 2018 at 10:16 PM Stephen Smalley <sds@tycho.nsa.gov> wrote:
> > > >
> > > > On 11/29/18 4:03 PM, Stephen Smalley wrote:
> > > > > On 11/29/18 2:47 PM, Miklos Szeredi wrote:
> > > > > > On Thu, Nov 29, 2018 at 5:14 PM Stephen Smalley <sds@tycho.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?
> > > > >
> > > > > That's correct. The overlay inodes are all assigned the label from the
> > > > > context= mount option, and so are any upper inodes created through the
> > > > > overlay. At least that's my understanding of how it is supposed to
> > > > > work. The original use case was for containers with the lower dir
> > > > > labeled with a context that is read-only to the container context and
> > > > > using a context that is writable by the container context for the
> > > > > context= mount.
> > > > >
> > > > > > Next question: permission to change metadata is tied to permission to
> > > > > > open? Is it possible that open is denied, but metadata can be
> > > > > > changed?
> > > > >
> > > > > There is no metadata change occurring here. The overlay, upper, and
> > > > > lower inodes all keep their labels intact for their lifetime (both
> > > > > overlay and upper always have the context= label; upper has whatever its
> > > > ^^lower^^
> > > >
> > > > > original label was), unless explicitly relabeled by some process. And
> > > > > when viewed through the overlay, the file always has the label specified
> > > > > via context=, even before the copy-up.
> > >
> > > Okay.
> > >
> > > > >
> > > > > > 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?
> > > > >
> > > > > Actually, I guess there wouldn't be a privilege escalation if you
> > > > > checked the mounter's ability to create the new file upon copy-up, and
> > > > > checked the mounter's access to the upper inode label upon the
> > > > > subsequent read, write, or execute access. Then we'd typically block
> > > > > the ability to create the device file and we'd block the ability to
> > > > > execute files with the label from context=.
> > > > >
> > > > > But copy-up of special files seems undesirable for other reasons (e.g.
> > > > > requiring mounters to be allowed to create device nodes just to permit
> > > > > client's to read/write them, possible implications for nodev/noexec,
> > > > > implications for socket and fifo files).
> > >
> > > I think you missed my point: opening a device file or executing an
> > > executable wouldn't normally require copy-up. If
> > >
> > > - permission is granted on overlay to task, and
> > > - permission is granted on lower layer to mounter,
> > >
> > > then copy-up wouldn't be performed.
> > >
> > > My proposed sequence would be
> > >
> > > a) check task's creds against overlay inode, fail -> return fail, otherwise:
> > > b) check mounter's creds against lower inode, success -> return
> > > success, otherwise:
> > > c) copy up inode, fail -> return fail, otherwise
> > > d) check mounter's creds against upper inode, return result.
> > >
> > > So, unlike write access to regular files, write access to special
> > > files don't necessarily result in copy-up.
> > >
> > > You say this is an escalation of privilege, but I don't get it how.
> > > As DWalsh points out downthread, if mounter cannot create device
> > > files, then the copy-up will simply fail. If mounter can create
> > > device files, then this is not an escalation of privilege for the
> > > mounter.
> >
> > Yes, in that case there isn't an escalation of privilege for the mounter (I
> > acknowledged that above). I'm still not sure copy-up of special files is a
> > good idea though:
> >
> > - In the case of device files, there is the potential for mischief by the
> > client task in misusing the mounter's privileges to gain access to otherwise
> > unusable device node (nodev lower vs upper?),
>
> I was thinking about it as well. But client can always bypass permissions
> of lower device inode by first removing device file and then by doing
> a mknod. And that will be equivalent of copy up. IOW, IIUC, we do not deny
> mknod to client and that always creates a way for it to write to device
> file (and it does not matter what are permissions on lower?)
Having said that, this still create little anomaly when mknod to client
is not allowed on context label. So a device file, which is on lower
and client can not open it for read/write on host, it can now be opened
for read/write because mounter will allow access. So why it is different
that regular copy up. Well, in regular copy up, we created a copy of
the original object and allowed writing to that object (cp --preserve=all)
model. But in case of device file, writes will go to same original
object. (And not a separate copy).
Thanks
Vivek
next prev parent reply other threads:[~2018-12-04 15:22 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-27 19:55 overlayfs access checks on underlying layers Miklos Szeredi
2018-11-27 19:58 ` Miklos Szeredi
2018-11-27 21:05 ` Vivek Goyal
2018-11-28 10:00 ` Miklos Szeredi
2018-11-28 17:03 ` Vivek Goyal
2018-11-28 19:34 ` Stephen Smalley
2018-11-28 20:24 ` Miklos Szeredi
2018-11-28 21:46 ` Stephen Smalley
2018-11-29 11:04 ` Miklos Szeredi
2018-11-29 13:49 ` Vivek Goyal
2019-03-04 17:01 ` Mark Salyzyn
2019-03-04 17:56 ` Casey Schaufler
2019-03-04 18:44 ` Stephen Smalley
2019-03-04 19:21 ` Amir Goldstein
2018-11-29 16:16 ` Stephen Smalley
2018-11-29 16:22 ` Stephen Smalley
2018-11-29 19:47 ` Miklos Szeredi
2018-11-29 21:03 ` Stephen Smalley
2018-11-29 21:19 ` Stephen Smalley
2018-12-04 13:32 ` Miklos Szeredi
2018-12-04 14:30 ` Stephen Smalley
2018-12-04 14:45 ` Miklos Szeredi
2018-12-04 15:35 ` Stephen Smalley
2018-12-04 15:39 ` Miklos Szeredi
2018-12-11 15:50 ` Paul Moore
2018-12-04 15:15 ` Vivek Goyal
2018-12-04 15:22 ` Vivek Goyal [this message]
2018-12-04 15:31 ` Miklos Szeredi
2018-12-04 15:42 ` Vivek Goyal
2018-12-04 16:05 ` Stephen Smalley
2018-12-04 16:17 ` Vivek Goyal
2018-12-04 16:49 ` Stephen Smalley
2018-12-05 13:43 ` Vivek Goyal
2018-12-06 20:26 ` Stephen Smalley
2018-12-11 21:48 ` Vivek Goyal
2018-12-12 14:51 ` Stephen Smalley
2018-12-13 14:58 ` Vivek Goyal
2018-12-13 16:12 ` Stephen Smalley
2018-12-13 18:54 ` Vivek Goyal
2018-12-13 20:09 ` Stephen Smalley
2018-12-13 20:26 ` Vivek Goyal
2018-12-04 15:42 ` Stephen Smalley
2018-12-04 16:15 ` Vivek Goyal
2018-11-29 22:22 ` Daniel Walsh
2018-12-03 23:27 ` Paul Moore
2018-12-04 14:43 ` Stephen Smalley
2018-12-04 23:01 ` Paul Moore
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