On Fri, Aug 20, 2021 at 06:33:38AM +0200, Jordy Zomer wrote: > When a secret memory region is active, memfd_secret disables > hibernation. One of the goals is to keep the secret data from being > written to persistent-storage. > > It accomplishes this by maintaining a reference count to > `secretmem_users`. Once this reference is held your system can not be > hibernated due to the check in `hibernation_available()`. However, > because `secretmem_users` is of type `atomic_t`, reference counter > overflows are possible. It's an unlikely condition to hit given max-open-fds, etc, but there's no reason to leave this weakness. Changing this to refcount_t is easy and better than using atomic_t. Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx> > As you can see there's an `atomic_inc` for each `memfd` that is opened > in the `memfd_secret` syscall. If a local attacker succeeds to open 2^32 > memfd's, the counter will wrap around to 0. This implies that you may > hibernate again, even though there are still regions of this secret > memory, thereby bypassing the security check. IMO, this hibernation check is also buggy, since it looks to be vulnerable to ToCToU: processes aren't frozen when hibernation_available() checks secretmem_users(), so a process could add one and fill it before the process freezer stops it. And of course, there's still the ptrace hole[1], which is think is quite serious as it renders the entire defense moot. -Kees [1] https://lore.kernel.org/lkml/202105071620.E834B1FA92@keescook/ -- Kees Cook
