> From: Mimi Zohar [mailto:zohar@xxxxxxxxxxxxx] > Sent: Tuesday, September 1, 2020 1:05 PM > On Tue, 2020-09-01 at 09:08 +0000, Roberto Sassu wrote: > > > From: Mimi Zohar [mailto:zohar@xxxxxxxxxxxxx] > > > Sent: Monday, August 24, 2020 2:18 PM > > > On Thu, 2020-06-18 at 18:04 +0200, Roberto Sassu wrote: > > > > When EVM_ALLOW_METADATA_WRITES is set, EVM allows any > operation > > > on > > > > metadata. Its main purpose is to allow users to freely set metadata > when > > > > they are protected by a portable signature, until the HMAC key is > loaded. > > > > > > > > However, IMA is not notified about metadata changes and, after the > first > > > > appraisal, always allows access to the files without checking metadata > > > > again. > > > > > > ^after the first successful appraisal > > > > > > > > This patch checks in evm_reset_status() if EVM_ALLOW_METADATA > > > WRITES is > > > > enabled and if it is, sets the IMA_CHANGE_XATTR/ATTR bits > depending on > > > the > > > > operation performed. At the next appraisal, metadata are revalidated. > > > > > > EVM modifying IMA bits crosses the boundary between EVM and IMA. > > > There > > > is already an IMA post_setattr hook. IMA could reset its own bit > > > there. If necessary EVM could export as a function it's status info. > > > > I wouldn't try to guess in IMA when EVM resets its status. We would have > > to duplicate the logic to check if an EVM key is loaded, if the passed xattr > > is a POSIX ACL, ... > > Agreed, but IMA could call an EVM function. > > > > > I think it is better to set a flag, maybe a new one, directly in EVM, to notify > > the integrity subsystem that iint->evm_status is no longer valid. > > > > If the EVM flag is set, IMA would reset the appraisal flags, as it uses > > iint->evm_status for appraisal. We can consider to reset also the measure > > flags when we have a template that includes file metadata. > > When would IMA read the EVM flag? Who would reset the flag? At what > point would it be reset? Just as EVM shouldn't be resetting the IMA > flag, IMA shouldn't be resetting the EVM flag. IMA would read the flag in process_measurement() and behave similarly to when it processes IMA_CHANGE_ATTR. The flag would be reset by evm_verify_hmac(). Roberto HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063 Managing Director: Li Peng, Li Jian, Shi Yanli
