On 9/1/2020 11:11 AM, Dave Hansen wrote:
On 9/1/20 10:45 AM, Andy Lutomirski wrote:
For arm64 (and sparc etc.) we continue to use the regular mmap/mprotect
family of calls. One or two additional arch-specific mmap flags are
sufficient for now.
Is x86 definitely not going to fit within those calls?
That can work for x86. Andy, what if we create PROT_SHSTK, which can
been seen only from the user. Once in kernel, it is translated to
VM_SHSTK. One question for mremap/mprotect is, do we allow a normal
data area to become shadow stack?
I'm unconvinced that we want to use a somewhat precious PROT_ or VM_
bit for this. Using a flag bit makes sense if we expect anyone to
ever map an fd or similar as a shadow stack, but that seems a bit odd
in the first place. To me, it seems more logical for a shadow stack
to be a special sort of mapping with a special vm_ops, not a normal
mapping with a special flag set. Although I realize that we want
shadow stacks to work like anonymous memory with respect to fork().
Dave?
I actually don't like the idea of *creating* mappings much.
I think the pkey model has worked out pretty well where we separate
creating the mapping from doing something *to* it, like changing
protections. For instance, it would be nice if we could preserve things
like using hugetlbfs or heck even doing KSM for shadow stacks.
If we're *creating* mappings, we've pretty much ruled out things like
hugetlbfs.
Something like mprotect_shstk() would allow an implementation today that
only works on anonymous memory *and* sets up a special vm_ops. But, the
same exact ABI could do wonky stuff in the future if we decided we
wanted to do shadow stacks on DAX or hugetlbfs or whatever.
I don't really like the idea of PROT_SHSTK those are plumbed into a
bunch of interfaces. But, I also can't deny that it seems to be working
fine for the arm64 folks.
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.
Yu-cheng
