On 9/8/2020 10:57 AM, Dave Hansen wrote:
On 9/8/20 10:50 AM, Yu, Yu-cheng wrote:
What about this:
- Do not add any new syscall or arch_prctl for creating a new shadow stack.
- Add a new arch_prctl that can turn an anonymous mapping to a shadow
stack mapping.
This allows the application to do whatever is necessary. It can even
allow GDB or JIT code to create or fix a call stack.
Fine with me. But, it's going to effectively be
arch_prctl(PR_CONVERT_TO_SHS..., addr, len);
when it could just as easily be:
madvise(addr, len, MADV_SHSTK...);
Or a new syscall. The only question in my mind is whether we want to do
something generic that we can use for other similar things in the
future, like:
madvise2(addr, len, flags, MADV2_SHSTK...);
I don't really feel strongly about it, though. Could you please share
your logic on why you want a prctl() as opposed to a whole new syscall?
A new syscall is more intrusive, I think. When creating a new shadow
stack, the kernel also installs a restore token on the top of the new
shadow stack, and it is somewhat x86-specific. So far no other arch's
need this.
Yes, madvise is better if the kernel only needs to change the mapping.
The application itself can create the restore token before calling
madvise().
