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    SubjectRe: [PATCH v3 0/7] User namespace mount updates
    Quoting Theodore Ts'o (tytso@mit.edu):
    > On Tue, Nov 17, 2015 at 12:34:44PM -0600, Seth Forshee wrote:
    > > On Tue, Nov 17, 2015 at 05:55:06PM +0000, Al Viro wrote:
    > > > On Tue, Nov 17, 2015 at 11:25:51AM -0600, Seth Forshee wrote:
    > > >
    > > > > Shortly after that I plan to follow with support for ext4. I've been
    > > > > fuzzing ext4 for a while now and it has held up well, and I'm currently
    > > > > working on hand-crafted attacks. Ted has commented privately (to others,
    > > > > not to me personally) that he will fix bugs for such attacks, though I
    > > > > haven't seen any public comments to that effect.
    > > >
    > > > _Static_ attacks, or change-image-under-mounted-fs attacks?
    > >
    > > Right now only static attacks, change-image-under-mounted-fs attacks
    > > will be next.
    >
    > I will fix bugs about static attacks. That is, it's interesting to me
    > that a buggy file system (no matter how it is created), not cause the
    > kernel to crash --- and privilege escalation attacks tend to be
    > strongly related to those bugs where we're not doing strong enough
    > checking.
    >
    > Protecting against a malicious user which changes the image under the
    > file system is a whole other kettle of fish. I am not at all user you
    > can do this without completely sacrificing performance or making the
    > code impossible to maintain. So my comments do *not* extend to
    > protecting against a malicious user who is changing the block device
    > underneath the kernel.

    Yup, thanks, Ted. I think the only sane thing to do is work on making the
    mounted files immutable. Guarding against under-mounted-writes seems
    crazy. Well, actually it seems like a fascinating problem, and maybe
    solvable without fs changes, but not in scope here.

    > If you want to submit patches to make the kernel more robust against
    > these attacks, I'm certainly willing to look at the patches. But I'm
    > certainly not guaranteeing that they will go in, and I'm certainly not
    > promising to fix all vulnerabilities that you might find that are
    > caused by a malicious block device. Sorry, that's too much buying a
    > pig in a poke....
    >
    > - Ted
    >


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