Hi, Am Dienstag, 12. Februar 2019, 04:29:30 CET schrieb Amir Goldstein: > On Mon, Feb 11, 2019 at 10:28 PM Ignaz Forster <iforster@xxxxxxxx> wrote: > > > > From: Ignaz Forster <iforster@xxxxxxx> > > > > This patch series tries to solve several problems found when using > > EVM on an overlay file system. > > > > Especially patch 4 and 5 will need further discussion; patch 4 will > > introduce follow up problems, patch 5 can be considered a workaround > > at best. > > > > I think maybe the entire series (short of vfs_create hook) is a workaround. Correct. > Let's stop and think. What is IMA/EVM meant to do? > Provide tamper proof protection for persistent storage. Right? > > Overlayfs uses other filesystems to store persistent data/metadata, > so if IMA/EVM already protect those other filesystems, we got > tamper proof already don't we? Yup. There are two approaches overlayfs can interface with IMA/EVM: a) (vfs) -> IMA/EVM -> overlayfs -> filesystem b) (vfs) -> overlayfs -> IMA/EVM -> filesystem Both are from my view equally valid, just with different issues. Currently the first approach is used, as all IMA/EVM and security hooks are invoked from the "high-level" vfs functions and overlayfs calls the lower functions to interface with the underlying filesystems, skipping all those checks/processes. Doing b) has the disadvantage that overlayfs needs to become IMA/EVM-aware (or use the high-level functions for everything) and special care needs to be taken to a) do IMA/EVM work properly b) not do it twice. It would avoid an entire class of IMA/EVM issues though, by only doing work on the actual filesystem nodes and not on the virtual overlayfs ones. > The issue with overlayfs and security hooks is often credentials > because underlying filesystems are accessed with different > credentials (mounter credentials) than the overlay file access. > Is that an issue for IMA/EVM? I'm not familiar enough with IMA/EVM internals to comment here, but I assume that IMA/EVM is only invoked after a related operation succeeded, e.g. security.ima is only updated after a successful write. So this shouldn't be an issue (?). We could do some tests, but I'm not sure what we would need to look out for. > Or is it an issue that IMA policy is path based and may only > apply to the overlay path and not underlying path?? I'm not quite sure what you mean by that - I've never seen any paths involved in the IMA policy (/sys/kernel/security/ima/policy). > I had already suggested once to mark overlay inodes with > NOIMA flag: > https://marc.info/?l=linux-unionfs&m=154529013929472&w=2 > and there was a similar suggestion for FUSE: > https://marc.info/?l=linux-fsdevel&m=151871326115716&w=2 That explains why it was also broken with unionfs when I tried that briefly, I expected that FUSE mounts would just ignore IMA and due to access from userspace work properly with IMA/EVM OOTB as well - apparently not yet. > If my assumptions so far are correct, then the effort for making > IMA/EVM work with overlayfs should focus around finding the > places where overlayfs uses lower level vfs interface (often > vfs_xxx helpers) and make sure that the IMA hooks are place > in those lower vfs interfaces, just like vfs_create() patch does > and like vfs_tmpfile() patch did before it. So basically turning on NOIMA for overlayfs while ensuring that integrity checks and operations still perform as expected? Thanks, Fabian > Thanks, > Amir.
