> From: Mimi Zohar [mailto:zohar@xxxxxxxxxxxxx] > Sent: Friday, December 4, 2020 2:05 PM > On Fri, 2020-12-04 at 08:05 +0000, Roberto Sassu wrote: > > > From: Mimi Zohar [mailto:zohar@xxxxxxxxxxxxx] > > > Sent: Thursday, December 3, 2020 9:43 PM > > > Hi Roberto, > > > > > > On Wed, 2020-11-11 at 10:22 +0100, Roberto Sassu wrote: > > > > When a file is being created, LSMs can set the initial label with the > > > > inode_init_security hook. If no HMAC key is loaded, the new file will > have > > > > LSM xattrs but not the HMAC. > > > > > > > > Unfortunately, EVM will deny any further metadata operation on new > > > files, > > > > as evm_protect_xattr() will always return the INTEGRITY_NOLABEL > error. > > > This > > > > would limit the usability of EVM when only a public key is loaded, as > > > > commands such as cp or tar with the option to preserve xattrs won't > work. > > > > > > > > Ignoring this error won't be an issue if no HMAC key is loaded, as the > > > > inode is locked until the post hook, and EVM won't calculate the HMAC > on > > > > metadata that wasn't previously verified. Thus this patch checks if an > > > > HMAC key is loaded and if not, ignores INTEGRITY_NOLABEL. > > > > > > I'm not sure what problem this patch is trying to solve. > > > evm_protect_xattr() is only called by evm_inode_setxattr() and > > > evm_inode_removexattr(), which first checks whether > > > EVM_ALLOW_METADATA_WRITES is enabled. > > > > The idea is to also support EVM verification when only a public key > > is loaded. An advantage to do that is that for example we can prevent > > accidental metadata changes when the signature is portable. > > Right, there are a couple of scenarios. Let's be more specific as to > which scenario this patch is addressing. > > - a public key is loaded and EVM_ALLOW_METADATA_WRITES is enabled, > - a public key is loaded and EVM_ALLOW_METADATA_WRITES is disabled, > - an HMAC key is loaded > > For the first and last case, this patch shouldn't be necessary. Only > the second case, with EVM_ALLOW_METADATA_WRITES disabled, probably > does > not work. I would claim that is working as designed. If there is no HMAC key loaded and a file is created, I think EVM should not expect an HMAC and return an error. If we do metadata verification only when an HMAC key is loaded, we miss a functionality that could be useful also when only a public key is loaded. Roberto HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063 Managing Director: Li Peng, Li Jian, Shi Yanli
