On Tue, 2021-04-27 at 15:57 +0000, Roberto Sassu wrote: > > From: Mimi Zohar [mailto:zohar@xxxxxxxxxxxxx] > > Sent: Tuesday, April 27, 2021 5:35 PM > > On Tue, 2021-04-27 at 09:25 +0000, Roberto Sassu wrote: > > > > From: Mimi Zohar [mailto:zohar@xxxxxxxxxxxxx] > > > > Sent: Monday, April 26, 2021 9:49 PM > > > > On Fri, 2021-03-05 at 09:30 -0800, Casey Schaufler wrote: > > > > > > > However ... > > > > > > > > > > The special casing of IMA and EVM in security.c is getting out of > > > > > hand, and appears to be unnecessary. By my count there are 9 IMA > > > > > hooks and 5 EVM hooks that have been hard coded. Adding this IMA > > > > > hook makes 10. It would be really easy to register IMA and EVM as > > > > > security modules. That would remove the dependency they currently > > > > > have on security sub-system approval for changes like this one. > > > > > I know there has been resistance to "IMA as an LSM" in the past, > > > > > but it's pretty hard to see how it wouldn't be a win. > > > > It sholdn't be one way. Are you willing to also make the existing > > IMA/EVM hooks that are not currently security hooks, security hooks > > too? And accept any new IMA/EVM hooks would result in new security > > hooks? Are you also willing to add dependency tracking between LSMs? > > I already have a preliminary branch where IMA/EVM are full LSMs. > > Indeed, the biggest problem would be to have the new hooks > accepted. I can send the patch set for evaluation to see what > people think. Defining new security hooks is pretty straight forward. Perhaps at least wait until Casey responds before posting the patches. > > > > > Somehow I missed the new "lsm=" boot command line option, which > > > > dynamically allows enabling/disabling LSMs, being upstreamed. This > > > > would be one of the reasons for not making IMA/EVM full LSMs. > > > > > > Hi Mimi > > > > > > one could argue why IMA/EVM should receive a special > > > treatment. I understand that this was a necessity without > > > LSM stacking. Now that LSM stacking is available, I don't > > > see any valid reason why IMA/EVM should not be managed > > > by the LSM infrastructure. > > > > > > > Both IMA and EVM file data/metadata is persistent across boots. If > > > > either one or the other is not enabled the file data hash or file > > > > metadata HMAC will not properly be updated, potentially preventing the > > > > system from booting when re-enabled. Re-enabling IMA and EVM would > > > > require "fixing" the mutable file data hash and HMAC, without any > > > > knowledge of what the "fixed" values should be. Dave Safford referred > > > > to this as "blessing" the newly calculated values. > > > > > > IMA/EVM can be easily disabled in other ways, for example > > > by moving the IMA policy or the EVM keys elsewhere. > > > > Dynamically disabling IMA/EVM is very different than removing keys and > > preventing the system from booting. Restoring the keys should result > > in being able to re-boot the system. Re-enabling IMA/EVM, requires re- > > labeling the filesystem in "fix" mode, which "blesses" any changes made > > when IMA/EVM were not enabled. > > Uhm, I thought that if you move the HMAC key for example > and you boot the system, you invalidate all files that change, > because the HMAC is not updated. More likely you wouldn't be able to boot the system without the HMAC key. Mimi > > > > Also other LSMs rely on a dynamic and persistent state > > > (for example for file transitions in SELinux), which cannot be > > > trusted anymore if LSMs are even temporarily disabled. > > > > Your argument is because this is a problem for SELinux, make it also a > > problem for IMA/EVM too?! ("Two wrongs make a right") > > To me it seems reasonable to give the ability to people to > disable the LSMs if they want to do so, and at the same time > to try to prevent accidental disable when the LSMs should be > enabled. > > > > If IMA/EVM have to be enabled to prevent misconfiguration, > > > I think the same can be achieved if they are full LSMs, for > > > example by preventing that the list of enabled LSMs changes > > > at run-time. > > > > That ship sailed when "security=" was deprecated in favor of "lsm=" > > support, which dynamically enables/disables LSMs at runtime. > > Maybe this possibility can be disabled with a new kernel option. > I will think a more concrete solution. > > Roberto > > HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063 > Managing Director: Li Peng, Li Jian, Shi Yanli
