On Fri, 2023-04-07 at 09:29 -0400, Jeff Layton wrote: > > > > > > > > > > I would ditch the original proposal in favor of this 2-line patch shown here: > > > > > > > > > > https://lore.kernel.org/linux-integrity/a95f62ed-8b8a-38e5-e468-ecbde3b221af@xxxxxxxxxxxxx/T/#m3bd047c6e5c8200df1d273c0ad551c645dd43232 > > > > We should cool it with the quick hacks to fix things. :) > > > > Yeah. It might fix this specific testcase, but I think the way it uses > the i_version is "gameable" in other situations. Then again, I don't > know a lot about IMA in this regard. > > When is it expected to remeasure? If it's only expected to remeasure on > a close(), then that's one thing. That would be a weird design though. Historical background: Prior to IMA being upstreamed there was a lot of discussion about how much/how frequently to measure files. Re-measuring files after each write would impact performance. Instead of re-measuring files after each write, if a file already opened for write was opened for read (open writers) or a file already opened for read was opened for write (Time of Measure/Time of Use) the IMA meausrement list was invalidated by including a violation record in the measurement list. Only the BPRM hook prevents a file from being opened for write. > > > > > > > > > > > > > > > > > > > Ok, I think I get it. IMA is trying to use the i_version from the > > > > overlayfs inode. > > > > > > > > I suspect that the real problem here is that IMA is just doing a bare > > > > inode_query_iversion. Really, we ought to make IMA call > > > > vfs_getattr_nosec (or something like it) to query the getattr routine in > > > > the upper layer. Then overlayfs could just propagate the results from > > > > the upper layer in its response. > > > > > > > > That sort of design may also eventually help IMA work properly with more > > > > exotic filesystems, like NFS or Ceph. > > > > > > > > > > > > > > > > > > Maybe something like this? It builds for me but I haven't tested it. It > > > looks like overlayfs already should report the upper layer's i_version > > > in getattr, though I haven't tested that either: > > > > > > -----------------------8<--------------------------- > > > > > > [PATCH] IMA: use vfs_getattr_nosec to get the i_version > > > > > > IMA currently accesses the i_version out of the inode directly when it > > > does a measurement. This is fine for most simple filesystems, but can be > > > problematic with more complex setups (e.g. overlayfs). > > > > > > Make IMA instead call vfs_getattr_nosec to get this info. This allows > > > the filesystem to determine whether and how to report the i_version, and > > > should allow IMA to work properly with a broader class of filesystems in > > > the future. > > > > > > Reported-by: Stefan Berger <stefanb@xxxxxxxxxxxxx> > > > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx> > > > --- > > > > So, I think we want both; we want the ovl_copyattr() and the > > vfs_getattr_nosec() change: > > > > (1) overlayfs should copy up the inode version in ovl_copyattr(). That > > is in line what we do with all other inode attributes. IOW, the > > overlayfs inode's i_version counter should aim to mirror the > > relevant layer's i_version counter. I wouldn't know why that > > shouldn't be the case. Asking the other way around there doesn't > > seem to be any use for overlayfs inodes to have an i_version that > > isn't just mirroring the relevant layer's i_version. > > It's less than ideal to do this IMO, particularly with an IS_I_VERSION > inode. > > You can't just copy up the value from the upper. You'll need to call > inode_query_iversion(upper_inode), which will flag the upper inode for a > logged i_version update on the next write. IOW, this could create some > (probably minor) metadata write amplification in the upper layer inode > with IS_I_VERSION inodes. > > > > (2) Jeff's changes for ima to make it rely on vfs_getattr_nosec(). > > Currently, ima assumes that it will get the correct i_version from > > an inode but that just doesn't hold for stacking filesystem. > > > > While (1) would likely just fix the immediate bug (2) is correct and > > _robust_. If we change how attributes are handled vfs_*() helpers will > > get updated and ima with it. Poking at raw inodes without using > > appropriate helpers is much more likely to get ima into trouble. > > This will fix it the right way, I think (assuming it actually works), > and should open the door for IMA to work properly with networked > filesystems that support i_version as well. On a local filesystem, there are guarantees that the calculated file hash is that of the file being used. Reminder IMA reads a file, page size chunk at a time into a single buffer, calculating the file hash. Once the file hash is calculated, the memory is freed. There are no guarantees on a fuse filesystem, for example, that the original file read and verified is the same as the one being executed. I'm not sure that the integrity guarantees of a file on a remote filesystem will be the same as those on a local file system. > > Note that there Stephen is correct that calling getattr is probably > going to be less efficient here since we're going to end up calling > generic_fillattr unnecessarily, but I still think it's the right thing > to do. > > If it turns out to cause measurable performance regressions though, > maybe we can look at adding a something that still calls ->getattr if it > exists but only returns the change_cookie value. Sure. For now, Reviewed-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>
