On Fri, Jul 17, 2015 at 12:47:35PM +1000, Dave Chinner wrote: > On Thu, Jul 16, 2015 at 07:42:03PM -0500, Eric W. Biederman wrote: > > Dave Chinner <da...@fromorbit.com> writes: > > > > > On Wed, Jul 15, 2015 at 11:47:08PM -0500, Eric W. Biederman wrote: > > >> Casey Schaufler <ca...@schaufler-ca.com> writes: > > >> > On 7/15/2015 6:08 PM, Andy Lutomirski wrote: > > >> >> If I mount an unprivileged filesystem, then either the contents were > > >> >> put there *by me*, in which case letting me access them are fine, or > > >> >> (with Seth's patches and then some) I control the backing store, in > > >> >> which case I can do whatever I want regardless of what LSM thinks. > > >> >> > > >> >> So I don't see the problem. Why would Smack or any other LSM care at > > >> >> all, unless it wants to prevent me from mounting the fs in the first > > >> >> place? > > >> > > > >> > First off, I don't cotton to the notion that you should be able > > >> > to mount filesystems without privilege. But it seems I'm being > > >> > outvoted on that. I suspect that there are cases where it might > > >> > be safe, but I can't think of one off the top of my head. > > >> > > >> There are two fundamental issues mounting filesystems without privielge, > > >> by which I actually mean mounting filesystems as the root user in a user > > >> namespace. > > >> > > >> - Are the semantics safe. > > >> - Is the extra attack surface a problem. > > > > > > I think the attack surface this exposes is the biggest problem > > > facing this proposal. > > > > I completely agree. > > > > >> Figuring out how to make semantics safe is what we are talking about. > > >> > > >> Once we sort out the semantics we can look at the handful of filesystems > > >> like fuse where the extra attack surface is not a concern. > > >> > > >> With that said desktop environments have for a long time been > > >> automatically mounting whichever filesystem you place in your computer, > > >> so in practice what this is really about is trying to align the kernel > > >> with how people use filesystems. > > > > > > The key difference is that desktops only do this when you physically > > > plug in a device. With unprivileged mounts, a hostile attacker > > > doesn't need physical access to the machine to exploit lurking > > > kernel filesystem bugs. i.e. they can just use loopback mounts, and > > > they can keep mounting corrupted images until they find something > > > that works. > > > > Yep. That magnifies the problem quite a bit. > > > > > User namespaces are supposed to provide trust separation. The > > > kernel filesystems simply aren't hardened against unprivileged > > > attacks from below - there is a trust relationship between root and > > > the filesystem in that they are the only things that can write to > > > the disk. Mounts from within a userns destroys this relationship as > > > the userns root, by definition, is not a trusted actor. > > > > I talked to Ted Tso a while back and ext4 is at least in principle > > already hardened against that kind of attack. I am not certain I > > believe it, but if it is true I think it is fantastic. > > No, it's not. No filesystem is, because to harden against such > attacks requires complete verification of all metadata when it is > read from disk, before it is used, or some method or ensuring the > block was not tampered with. CRCs are not sufficient, because they > can be tampered with, too. > > The only way a filesystem would be able to trust what it reads from > disk has not been tampered with in a system with untrusted mounts is > if it has some kind of cryptographically secure signature in the > metadata and the attacker is unable to access the key for that > signature.
Preventing tampering is a little different from protecting the kernel from attack, isn't it? I thought the latter was what people were asking about. So, for example, a screwed up on-disk directory structure shouldn't result in creating a cycle in the dcache and then deadlocking. --b. > No filesystem we have has that capability and AFAIA there > are no plans for any filesystem to implement such tamper detection. > And no, ext4 encryption does not provide this because it only stores > the values and data in encrypted format and does not protect > metadata from tampering when it is not mounted. > > If we don't have crypto signatures in metadata, then XFS is probably > the most robust against tampering as it does a lot more checking of > the on-disk metadata before it is used than any other filesystem > (i.e. see the verifier infrastructure that does corruption checks > after read (in io completion) and before write (in io submission) > to catch bad metadata before it is used by the kernel, or before it > is written to disk by the kernel. > > However, these checks are far from comprehensive. we can only check > internal consistency of the metadata objects in the block, and even > then we really only can check for values within range rather than > absolute correctness. e.g. we can check a dirent has a valid name, > length, ftype and inode number, but we can't validate that the inode > is actually allocated or not because that requires a lookup in the > allocated inode btree. We *trust* that inode number to be > allocated and valid because it is in metadata the filesystem wrote. > > For inode numbers that come from untrusted sources (NFS, > open-by-handle, etc) we have a flag that does inode number > validation on lookup (XFS_IGET_UNTRUSTED) to check against trusted > metadata (i.e. the allocated inode btrees), but that is expensive > and so not done on inodes that we pull directly from metadata that > has come from disk. Indeed, we still trust on-disk metadata to be > correct to validate that other metadata canbe trusted, so if one > structure can be tampered with, so can others. > > IOWs, if we cannot trust one part of the filesystem metadata to be > correct, then we cannot trust that filesystem *at all*, *for > anything*. And even running fsck doesn't restore trust - all it does > is tell us that any modification that was made is not a detectable > inconsistency that needs fixing. > > > At this point any setting of the FS_USER_MOUNT flag I figure needs to go > > through the filesystem maintainers tree and they need to be aware of and > > agree to deal with the attack from below issue. > > > > The one filesystem I truly expect we can make work is fuse. fuse has > > been designed to deal with some variation of the attack from below issue > > since day one. We looked at what the patches to fuse would look like > > with the current state of the vfs and it was not pretty. > > > > We very much need to sort through as much as possible at the vfs layer, > > and in generic code. Allow everyone to see what is going on and how > > it works before preceeding forward with enabling any filesystems. > > The VFS protects us from attacks from above the filesystem, not > below. The VFS plays no part in validating the on-disk structure of > a filesystem which is what attacks from below will be attempting to > exploit. > > > I truly hope we can find a small set of block device filesystems that we > > can harden from attack below. That would allow linux to have serious > > defenses against evil usb stick attacks. I think that is going to take > > a lot of careful coding, testing and validation and advancing the state > > of the art to get there. > > Somehow, I just can't see that happening. > > Cheers, > > Dave. > -- > Dave Chinner > da...@fromorbit.com > -- > To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in > the body of a message to majord...@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
