On Fri, Oct 06, 2023 at 07:29:45AM -0500, Eric W. Biederman wrote:
Mark Brown broonie@kernel.org writes:
It's not just the default size that I dislike (I think the x86 RLIMIT_STACK or clone3() stack_size is probably good enough) but the kernel allocating the shadow stack and inserting it into the user address space. The actual thread stack is managed by the user but the shadow stack is not (and we don't do this very often). Anyway, I don't have a better solution for direct uses of clone() or clone3(), other than running those threads with the shadow stack disabled. Not sure that's desirable.
Running threads with the shadow stack disabled if they don't explicitly request it feels like it's asking for trouble - as well as the escape route from the protection it'd provide I'd expect there to be trouble for things that do stack pivots, potentially random issues if there's a mix of ways threads are started. It's going to be a tradeoff whatever we do.
Something I haven't seen in the discussion is that one of the ways I have seen a non-libc clone used is to implement a fork with flags. That is a new mm is created, and effectively a new process. Which makes the characterization different.
In general creating a thread with clone and bypassing libc is incompatible with pthreads, and the caller gets to keep both pieces.
As long as there is enough information code can detect that shadow stacks are in use, and the code is able to create their own I don't see why it shouldn't be the callers responsibility.
On the other hand I don't see the maintainer of clone Christian Brauner or the libc folks especially Florian cc'd on this thread. So I really don't think you have the right folks in on this conversation.
Well, copying them in now. The discussion here is about allocation of shadow stacks for the arm64 implementation of the feature (the arm64 feature is called Guarded Control Stack in the architecture). These maintain a second copy of the stack with only the return targets in memory allocated with special protections so userspace can't write to it directly and use this when doing returns to ensure that the returns haven't been redirected. These shadow stacks can be allocated directly by userspace using a new system call map_shadow_stack(), doing this via mmap() was extensively discussed but it was concluded that this was very likely to lead to security problems so we've got this new syscall that ensures that shadow stack memory is never accessible to userspace via other means.
The x86 implementation that has already been merged into mainline will allocate a new shadow stack for newly created threads when the creating thread has one. There was a suggestion to have arm64 diverge and require that threads be created with clone3() and manualy provide a shadow stack but then concerns were raised that as well as the issues with divergence this would be too disruptive for adoption due to non-libc thread creation. It's not controversial that it'd be good to have clone3() by able to explicitly specify a shadow stack, just if it should be required.