On Fri, Dec 8, 2023 at 7:25 PM Roberto Sassu <roberto.sassu@xxxxxxxxxxxxxxx> wrote: > > From: Roberto Sassu <roberto.sassu@xxxxxxxxxx> > > EVM updates the HMAC in security.evm whenever there is a setxattr or > removexattr operation on one of its protected xattrs (e.g. security.ima). > > Unfortunately, since overlayfs redirects those xattrs operations on the > lower filesystem, the EVM HMAC cannot be calculated reliably, since lower > inode attributes on which the HMAC is calculated are different from upper > inode attributes (for example i_generation and s_uuid). > > Although maybe it is possible to align such attributes between the lower > and the upper inode, another idea is to map security.evm to another name > (security.evm_overlayfs) If we were to accept this solution, this will need to be trusted.overlay.evm to properly support private overlay xattr escaping. > during an xattr operation, so that it does not > collide with security.evm set by the lower filesystem. You are using wrong terminology and it is very confusing to me. see the overlay mount command has lowerdir= and upperdir=. Seems that you are using lower filesystem to refer to the upper fs and upper filesystem to refer to overlayfs. > > Whenever overlayfs wants to set security.evm, it is actually setting > security.evm_overlayfs calculated with the upper inode attributes. The > lower filesystem continues to update security.evm. > I understand why that works, but I am having a hard time swallowing the solution, mainly because I feel that there are other issues on the intersection of overlayfs and IMA and I don't feel confident that this addresses them all. If you want to try to convince me, please try to write a complete model of how IMA/EVM works with overlayfs, using the section "Permission model" in Documentation/filesystems/overlayfs.rst as a reference. Thanks, Amir.
