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. > > > "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
