On Mon, Dec 11, 2023 at 04:41:46PM +0100, Roberto Sassu wrote: > On Mon, 2023-12-11 at 09:36 -0600, Seth Forshee wrote: > > On Mon, Dec 11, 2023 at 03:56:06PM +0100, Roberto Sassu wrote: > > > Ok, I will try. > > > > > > I explain first how EVM works in general, and then why EVM does not > > > work with overlayfs. > > > > > > EVM gets called before there is a set/removexattr operation, and after, > > > if that operation is successful. Before the set/removexattr operation > > > EVM calculates the HMAC on current inode metadata (i_ino, i_generation, > > > i_uid, i_gid, i_mode, POSIX ACLs, protected xattrs). Finally, it > > > compares the calculated HMAC with the one in security.evm. > > > > > > If the verification and the set/removexattr operation are successful, > > > EVM calculates again the HMAC (in the post hooks) based on the updated > > > inode metadata, and sets security.evm with the new HMAC. > > > > > > The problem is the combination of: overlayfs inodes have different > > > metadata than the lower/upper inodes; overlayfs calls the VFS to > > > set/remove xattrs. > > > > I don't know all of the inner workings of overlayfs in detail, but is it > > not true that whatever metadata an overlayfs mount presents for a given > > inode is stored in the lower and/or upper filesystem inodes? If the > > metadata for those inodes is verified with EVM, why is it also necessary > > to verify the metadata at the overlayfs level? If some overlayfs > > metadata is currently omitted from the checks on the lower/upper inodes, > > is there any reason EVM couldn't start including that its checksums? > > Currently, the metadata where there is a misalignment are: > i_generation, s_uuid, (i_ino?). Maybe there is more? > > If metadata are aligned, there is no need to store two separate HMACs. I can only think of three possible sources for the metadata overlayfs presents: 1. It comes directly from the underlying filesystems 2. overlayfs synthesizes if from the underlying filesystem data 3. It's purely generated at runtime Are there others? 1 and 2 should be covered by EVM on the underlying filesystems. If 3 is happening then it seems like hashing that data is just confirming that overlayfs consistently generates the same values for that data, and verifying code behavior doesn't seem in-scope for EVM.
