On Tue, Jul 9, 2024 at 1:42 PM Mickaël Salaün <mic@xxxxxxxxxxx> wrote: > > On Mon, Jul 08, 2024 at 03:07:24PM -0700, Jeff Xu wrote: > > On Mon, Jul 8, 2024 at 2:25 PM Steve Dower <steve.dower@xxxxxxxxxx> wrote: > > > > > > On 08/07/2024 22:15, Jeff Xu wrote: > > > > IIUC: > > > > CHECK=0, RESTRICT=0: do nothing, current behavior > > > > CHECK=1, RESTRICT=0: permissive mode - ignore AT_CHECK results. > > > > CHECK=0, RESTRICT=1: call AT_CHECK, deny if AT_CHECK failed, no exception. > > > > CHECK=1, RESTRICT=1: call AT_CHECK, deny if AT_CHECK failed, except > > > > those in the "checked-and-allowed" list. > > > > > > I had much the same question for Mickaël while working on this. > > > > > > Essentially, "CHECK=0, RESTRICT=1" means to restrict without checking. > > > In the context of a script or macro interpreter, this just means it will > > > never interpret any scripts. Non-binary code execution is fully disabled > > > in any part of the process that respects these bits. > > > > > I see, so Mickaël does mean this will block all scripts. > > That is the initial idea. > > > I guess, in the context of dynamic linker, this means: no more .so > > loading, even "dlopen" is called by an app ? But this will make the > > execve() fail. > > Hmm, I'm not sure this "CHECK=0, RESTRICT=1" configuration would make > sense for a dynamic linker except maybe if we want to only allow static > binaries? > > The CHECK and RESTRICT securebits are designed to make it possible a > "permissive mode" and an enforcement mode with the related locked > securebits. This is why this "CHECK=0, RESTRICT=1" combination looks a > bit weird. We can replace these securebits with others but I didn't > find a better (and simple) option. I don't think this is an issue > because with any security policy we can create unusable combinations. > The three other combinations makes a lot of sense though. > If we need only handle 3 combinations, I would think something like below is easier to understand, and don't have wield state like CHECK=0, RESTRICT=1 XX_RESTRICT: when true: Perform the AT_CHECK, and deny the executable after AT_CHECK fails. XX_RESTRICT_PERMISSIVE: take effect when XX_RESTRICT is true. True means Ignoring the AT_CHECK result. Or XX_CHECK: when true: Perform the AT_CHECK. XX_CHECK_ENFORCE takes effect only when XX_CHECK is true. True means restrict the executable when AT_CHECK failed; false means ignore the AT_CHECK failure. Of course, we can replace XX_CHECK_ENFORCE with XX_RESTRICT. Personally I think having _CHECK_ in the name implies the XX_CHECK needs to be true as a prerequisite for this flag , but that is my opinion only. As long as the semantics are clear as part of the comments of definition in code, it is fine. Thanks -Jeff > > > > > "CHECK=1, RESTRICT=1" means to restrict unless AT_CHECK passes. This > > > case is the allow list (or whatever mechanism is being used to determine > > > the result of an AT_CHECK check). The actual mechanism isn't the > > > business of the script interpreter at all, it just has to refuse to > > > execute anything that doesn't pass the check. So a generic interpreter > > > can implement a generic mechanism and leave the specifics to whoever > > > configures the machine. > > > > > In the context of dynamic linker. this means: > > if .so passed the AT_CHECK, ldopen() can still load it. > > If .so fails the AT_CHECK, ldopen() will fail too. > > Correct > > > > > Thanks > > -Jeff > > > > > The other two case are more obvious. "CHECK=0, RESTRICT=0" is the > > > zero-overhead case, while "CHECK=1, RESTRICT=0" might log, warn, or > > > otherwise audit the result of the check, but it won't restrict execution. > > > > > > Cheers, > > > Steve
