> On Aug 11, 2020, at 2:28 PM, James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote: > > On Tue, 2020-08-11 at 10:48 -0400, Chuck Lever wrote: >> Mimi's earlier point is that any IMA metadata format that involves >> unsigned digests is exposed to an alteration attack at rest or in >> transit, thus will not provide a robust end-to-end integrity >> guarantee. > > I don't believe that is Mimi's point, because it's mostly not correct: > the xattr mechanism does provide this today. The point is the > mechanism we use for storing IMA hashes and signatures today is xattrs > because they have robust security properties for local filesystems that > the kernel enforces. This use goes beyond IMA, selinux labels for > instance use this property as well. I don't buy this for a second. If storing a security label in a local xattr is so secure, we wouldn't have any need for EVM. > What I think you're saying is that NFS can't provide the robust > security for xattrs we've been relying on, so you need some other > mechanism for storing them. For NFS, there's a network traversal which is an attack surface. A local xattr can be attacked as well: a device or bus malfunction can corrupt the content of an xattr, or a privileged user can modify it. How does that metadata get from the software provider to the end user? It's got to go over a network, stored in various ways, some of which will not be trusted. To attain an unbroken chain of provenance, that metadata has to be signed. I don't think the question is the storage mechanism, but rather the protection mechanism. Signing the metadata protects it in all of these cases. > I think Mimi's other point is actually that IMA uses a flat hash which > we derive by reading the entire file and then watching for mutations. > Since you cannot guarantee we get notice of mutation with NFS, the > entire IMA mechanism can't really be applied in its current form and we > have to resort to chunk at a time verifications that a Merkel tree > would provide. I'm not sure what you mean by this. An NFS client relies on notification of mutation to maintain the integrity of its cache of NFS file content, and it's done that since the 1980s. In addition to examining a file's mtime and ctime as maintained by the NFS server, a client can rely on the file's NFSv4 change attribute or an NFSv4 delegation. > Doesn't this make moot any thinking about > standardisation in NFS for the current IMA flat hash mechanism because > we simply can't use it ... If I were to construct a prototype I'd have > to work out and securely cache the hash of ever chunk when verifying > the flat hash so I could recheck on every chunk read. I think that's > infeasible for large files. > > James > -- Chuck Lever chucklever@xxxxxxxxx
