On Mon, Dec 11, 2023 at 7:15 PM Seth Forshee <sforshee@xxxxxxxxxx> wrote: > > 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? 3.b. purely generated and persisted in overlay private xattr but IIRC only s_uuid fits in that category > > 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. I agree. I don't think that IMA/EVM has a reason to assets overlayfs specific metadata. Thanks, Amir.
