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. Thanks Roberto > Granted that there could be some backwards compatibility issues, but > maybe inclusion of the overlayfs metadata could be opt-in. > > Thanks, > Seth
