Re: [PATCH v7 33/41] x86/shstk: Introduce map_shadow_stack syscall

[Szabolcs Nagy]

The 02/27/2023 14:29, Rick Edgecombe wrote:
> Previously, a new PROT_SHADOW_STACK was attempted,
...
> So rather than repurpose two existing syscalls (mmap, madvise) that don't
> quite fit, just implement a new map_shadow_stack syscall to allow
> userspace to map and setup new shadow stacks in one step. While ucontext
> is the primary motivator, userspace may have other unforeseen reasons to
> setup it's own shadow stacks using the WRSS instruction. Towards this
> provide a flag so that stacks can be optionally setup securely for the
> common case of ucontext without enabling WRSS. Or potentially have the
> kernel set up the shadow stack in some new way.
...
> The following example demonstrates how to create a new shadow stack with
> map_shadow_stack:
> void *shstk = map_shadow_stack(addr, stack_size, SHADOW_STACK_SET_TOKEN);

i think

mmap(addr, size, PROT_READ, MAP_ANON|MAP_SHADOW_STACK, -1, 0);

could do the same with less disruption to users (new syscalls
are harder to deal with than new flags). it would do the
guard page and initial token setup too (there is no flag for
it but could be squeezed in).

most of the mmap features need not be available (EINVAL) when
MAP_SHADOW_STACK is specified.

the main drawback is running out of mmap flags so extension
is limited. (but the new syscall has limitations too).