[RFC PATCH 2/4] ima: fail signature verification on unprivileged & untrusted filesystems

[Mimi Zohar]

On Wed, 2018-02-14 at 09:42 -0600, Serge E. Hallyn wrote:
> Quoting Mimi Zohar (zohar at linux.vnet.ibm.com):
> > On Wed, 2018-02-14 at 09:16 -0600, Serge E. Hallyn wrote:
> > > Quoting Mimi Zohar (zohar at linux.vnet.ibm.com):
> > > > On Wed, 2018-02-14 at 08:49 -0600, Serge E. Hallyn wrote:
> > > > > Quoting Mimi Zohar (zohar at linux.vnet.ibm.com):
> > > > > > Files on untrusted filesystems, such as fuse, can change at any time,
> > > > > > making the measurement(s) and by extension signature verification
> > > > > > meaningless.
> > > > > > 
> > > > > > FUSE can be mounted by unprivileged users either today with fusermount
> > > > > > installed with setuid, or soon with the upcoming patches to allow FUSE
> > > > > > mounts in a non-init user namespace.
> > > > > > 
> > > > > > This patch always fails the file signature verification on unprivileged
> > > > > > and untrusted filesystems.  To also fail file signature verification on
> > > > > 
> > > > > Why only untrusted?  Fuse could cause the same issue if it just
> > > > > messes up when mounted from init userns right?
> > > > 
> > > > Right, whether it is an unprivileged mount or not, fuse can return
> > > > whatever it wants, whenever it wants.  IMA can calculate the file hash
> > > > based based on what it reads, but fuse can return whatever it wants on
> > > > subsequent reads.
> > > 
> > > Ok but your patch seems to let privileged fuse mounts slide?  (see below)
> > 
> > Unprivileged fuse mounts hasn't been upstreamed yet, so we wouldn't be
> > breaking existing userspace.
> 
> I don't think I'm being clear.
> 
> In your patch it looks like you mark unprivileged FUSE mounts as
> INTEGRITY_FAIL.  I agree you should do that.  But you skip the
> FS_UNTRUSTED check for privileged FUSE mounts.  I'm asking why
> that's ok.
> 
> > > > Refer to the discussion with Linus - http://kernsec.org/pipermail/linu
> > > > x-security-module-archive/2018-February/005200.html
> > > > 
> > > > > > privileged, untrusted filesystems requires a custom policy.
> > > > > 
> > > > > (I'm not saying you shouldn't do this, but) does this mean that
> > > > > a container whose rootfs is fuse-mounted by the unprivileged user
> > > > > cannot possibly use IMA?
> > > > 
> > > > How would you suggest to differentiate between your unprivileged fuse
> > > > mounts from unintended, unintended malicious ones?
> > > 
> > > I wouldn't.
> > 
> > What happened to the requirement that systems should be "fail-safe"?
> 
> My point was - I was asking whether there was any way to have IMA be
> meaningful with such containers, not saying I had any ideas, and
> certainly not saying that just because you can't detect it means you
> should allow it in all cases.  It's too bad that it has this effect,
> but I agree with your patch.
> 
> I only didn't ack it because you're skipping the check for privileged
> mounts which seems wrong.

Oh!  That is based on Linus' "request" not to break userspace.

Mimi

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