On Tue, Nov 21, 2023 at 12:21:37PM +0000, Szabolcs Nagy wrote:
The 11/21/2023 11:17, Christian Brauner wrote:
I have a few questions that are probably me just not knowing much about shadow stacks so hopefully I'm not asking you write a thesis by accident:
One thing it feels like it's worth saying up front here is that shadow stacks are userspace memory with special permissions and instructions for access - they are mapped into the userspace address range and userspace can directly interact with them in restricted ways. For example there's some thought to using shadow stacks in unwinders since all the return addresses are stored in a single convenient block of memory which it's much harder to corrupt. Overflowing a shadow stack results in userspace getting a memory access fault just as with other memory access issues.
(2) With what other interfaces is implicit allocation and deallocation not consistent? I don't understand this argument. The kernel creates a shadow stack as a security measure to store return addresses. It seems to me exactly that the kernel should implicitly allocate and deallocate the shadow stack and not have userspace muck around with its size?
the kernel is not supposed to impose stack size policy or a particular programming model that limits the stack management options nor prevent the handling of stack overflows.
The inconsistency here is with the management of the standard stack - with the standard stack userspace passes an already allocated address range to the kernel. A constant tension during review of the shadow stack interfaces has been that shadow stack memory is userspace memory but the security constraints mean that we've come down on the side of having a custom allocation syscall for it instead of using flags on mmap() and friends like people often expect, and now having it allocated as part of clone3(). The aim is to highlight that this difference is deliberately chosen for specific reasons rather than just carelessness.
(3) Why is it safe for userspace to request the shadow stack size? What if they request a tiny shadow stack size? Should this interface require any privilege?
user can allocate huge or tiny stacks already.
and i think userspace can take control over shadow stack management: it can disable signals, start a clone child with stack_size == 1 page, map_shadow_stack and switch to it, enable signals. however this is complicated, leaks 1 page of kernel allocated shadow stack (+reserved guard page, i guess userspace could unmap, not sure if that works currently) and requires additional syscalls.
The other thing here is that if userspace gets this wrong it'll result in the userspace process hitting the top of the stack and getting fatal signals in a similar manner to what happens if it gets the size of the standard stack wrong (the kernel allocation does mean that there should always be guard pages and it's harder to overrun the stack and corrupt adjacent memory). There doesn't seem to be any meaningful risk here over what userspace can already do to itself anyway as part of thread allocation.
(4) Why isn't the @stack_size argument I added for clone3() enough? If it is specified can't the size of the shadow stack derived from it?
shadow stack only contains return addresses so it is proportional to the number of stack frames, not the stack size and it must account for sigaltstack too, not just the thread stack.
if you make minimal assumptions about stack usage and ignore the sigaltstack issue then the worst case shadow stack requirement is indeed proportional to the stack_size, but this upper bound can be pessimistic and userspace knows the tradeoffs better.
It's also worth pointing out here that the existing shadow stack support for x86 and in review code for arm64 make exactly these assumptions and guesses at a shadow stack size based on the stack_size for the thread. There's just been a general lack of enthusiasm for the fact that due to the need to store variables on the normal stack the resulting shadow stack is very likely to be substantially overallocated but we can't safely reduce the size without information from userspace.
And my current main objection is that shadow stacks were just released to userspace. There can't be a massive amount of users yet - outside of maybe early adopters.
no upstream libc has code to enable shadow stacks at this point so there are exactly 0 users in the open. (this feature requires runtime support)
the change is expected to allow wider deployability. (e.g. not just in glibc)
Right, and the lack of any userspace control of the shadow stack size has been a review concern with the arm64 GCS series which I'm trying to address here. The main concern is that userspaces that start a lot of threads are going to start using a lot more address space than they need to when shadow stacks are enabled. Given the fairly long deployment pipeline from extending a syscall to end users who might be using the feature in conjuction with imposing resource limits it does seem like a reasonable problem to anticipate.
The fact that there are other architectures that bring in a similar feature makes me even more hesitant. If they have all agreed _and_ implemented shadow stacks and have unified semantics then we can consider exposing control knobs to userspace that aren't implicitly architecture specific currently.
To be clear the reason I'm working on this is that I've implemented the arm64 support, I don't even have any access to x86 systems that have the feature (hence the RFT in the subject line) - Rick Edgecombe did the x86 work. The arm64 code is still in review, the userspace interface is very similar to that for x86 and there doesn't seem to be any controversy there which makes me expect that a change is likely. Unlike x86 we only have a spec and virtual implementations at present, there's no immintent hardware, so we're taking our time with making sure that everything is working well. Deepak Gupta (in CC) has been reviewing the series from the point of view of RISC-V. I think we're all fairly well aligned on the requirements here.