Re: [PATCH RFT v4 0/5] fork: Support shadow stacks in clone3()

[Mark Brown]

On Thu, Nov 30, 2023 at 07:00:58PM +0000, Catalin Marinas wrote:

> My hope when looking at the arm64 patches was that we can completely
> avoid the kernel allocation/deallocation of the shadow stack since it
> doesn't need to do this for the normal stack either. Could someone
> please summarise why we dropped the shadow stack pointer after v1? IIUC
> there was a potential security argument but I don't think it was a very
> strong one. Also what's the threat model for this feature? I thought
> it's mainly mitigating stack corruption. If some rogue code can do
> syscalls, we have bigger problems than clone3() taking a shadow stack
> pointer.

As well as preventing/detecting corruption of the in memory stack shadow
stacks are also ensuring that any return instructions are unwinding a
prior call instruction, and that the returns are done in opposite order
to the calls.  This forces usage of the stack - any value we attempt to
RET to is going to be checked against the top of the shadow stack which
makes chaining returns together as a substitute for branches harder.

The concern Rick raised was that allowing user to pick the exact shadow
stack pointer would allow userspace to corrupt or reuse the stack of an
existing thread by starting a new thread with the shadow stack pointing
into the existing shadow stack of that thread.  While in isolation
that's not too much more than what userspace could just do directly
anyway it might compose with other issues to something more "interesting"
(eg, I'd be a bit concerned about overlap with pkeys/POE though I've not
thought through potential uses in detail).

> I'm not against clone3() getting a shadow_stack_size argument but asking
> some more questions. If we won't pass a pointer as well, is there any
> advantage in expanding this syscall vs a specific prctl() option? Do we
> need a different size per thread or do all threads have the same shadow
> stack size? A new RLIMIT doesn't seem to map well though, it is more
> like an upper limit rather than a fixed/default size (glibc I think uses
> it for thread stacks but bionic or musl don't AFAIK).

I don't know what the userspace patterns are likely to be here, it's
possible a single value for each process might be fine but I couldn't
say that confidently.  I agree that a RLIMIT does seem like a poor fit.

As well as the actual configuration of the size the other thing that we
gain is that as well as relying on heuristics to determine if we need to
allocate a new shadow stack for the new thread we allow userspace to
explicitly request a new shadow stack.  There was some corner case with
IIRC posix_nspawn() mentioned where the heuristics aren't what we want
for example.

> Another dumb question on arm64 - is GCSPR_EL0 writeable by the user? If
> yes, can the libc wrapper for threads allocate a shadow stack via
> map_shadow_stack() and set it up in the thread initialisation handler
> before invoking the thread function?

No, GCSPR_EL0 can only be changed by EL0 through BL, RET and the
new GCS instructions (push/pop and stack switch).  Push is optional -
userspace has to explicitly request that it be enabled and this could be
prevented through seccomp or some other LSM.  The stack switch
instructions require a token at the destination address which must
either be written by a higher EL or will be written in the process of
switching away from a stack so you can switch back.  Unless I've missed
one every mechanism for userspace to update GCSPR_EL0 will do a GCS
memory access so providing guard pages have been allocated wrapping to a
different stack will be prevented.

We would need a syscall to allow GCSPR_EL0 to be written.

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