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

[Mike Rapoport]

Hi,

On Thu, Mar 09, 2023 at 10:55:11AM -0800, Deepak Gupta wrote:
> On Thu, Mar 02, 2023 at 05:22:07PM +0000, Szabolcs Nagy wrote:
> > 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).
> 
> Discussion on this topic in v6
> https://lore.kernel.org/all/20230223000340.GB945966@xxxxxxxxxxxxxxxxxxxxx/
> 
> Again I know earlier CET patches had protection flag and somehow due to pushback
> on mailing list, it was adopted to go for special syscall because no one else
> had shadow stack.
> 
> Seeing a response from Szabolcs, I am assuming arm4 would also want to follow
> using mmap to manufacture shadow stack. For reference RFC patches for risc-v shadow stack,
> use a new protection flag = PROT_SHADOWSTACK.
> https://lore.kernel.org/lkml/20230213045351.3945824-1-debug@xxxxxxxxxxxx/
> 
> I know earlier discussion had been that we let this go and do a re-factor later as other
> arch support trickle in. But as I thought more on this and I think it may just be
> messy from user mode point of view as well to have cognition of two different ways of
> creating shadow stack. One would be special syscall (in current libc) and another `mmap`
> (whenever future re-factor happens)
> 
> If it's not too late, it would be more wise to take `mmap`
> approach rather than special `syscall` approach.
 
I disagree. 

Having shadow stack flags for mmap() adds unnecessary complexity to the
core-mm, while having a dedicated syscall hides all the details in the
architecture specific code.

Another reason to use a dedicated system call allows for better
extensibility if/when we'd need to update the way shadow stack VMA is
created.

As for the userspace convenience, it is anyway required to add special
code for creating the shadow stack and it wouldn't matter if that code
would use mmap(NEW_FLAG) or map_shadow_stack().

> > 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).

-- 
Sincerely yours,
Mike.