Re: [PATCH v8 0/4] Introduce mseal

[Jeff Xu <jeffxu@chromium.org>]

On Fri, Feb 2, 2024 at 9:05 AM Theo de Raadt <deraadt@openbsd.org> wrote:
>
> Another interaction to consider is sigaltstack().
>
> In OpenBSD, sigaltstack() forces MAP_STACK onto the specified
> (pre-allocated) region, because on kernel-entry we require the "sp"
> register to point to a MAP_STACK region (this severely damages ROP pivot
> methods).  Linux does not have MAP_STACK enforcement (yet), but one day
> someone may try to do that work.
>
> This interacted poorly with mimmutable() because some applications
> allocate the memory being provided poorly.  I won't get into the details
> unless pushed, because what we found makes me upset.  Over the years,
> we've upstreamed diffs to applications to resolve all the nasty
> allocation patterns.  I think the software ecosystem is now mostly
> clean.
>
> I suggest someone in Linux look into whether sigaltstack() is a mseal()
> bypass, perhaps somewhat similar to madvise MADV_FREE, and consider the
> correct strategy.
>

Thanks for bringing this up. I will follow up on sigaltstack() in Linux.

> This is our documented strategy:
>
>      On OpenBSD some additional restrictions prevent dangerous address space
>      modifications.  The proposed space at ss_sp is verified to be
>      contiguously mapped for read-write permissions (no execute) and incapable
>      of syscall entry (see msyscall(2)).  If those conditions are met, a page-
>      aligned inner region will be freshly mapped (all zero) with MAP_STACK
>      (see mmap(2)), destroying the pre-existing data in the region.  Once the
>      sigaltstack is disabled, the MAP_STACK attribute remains on the memory,
>      so it is best to deallocate the memory via a method that results in
>      munmap(2).
>
> OK, I better provide the details of what people were doing.
> sigaltstacks() in .data, in .bss, using malloc(), on a buffer on the
> stack, we even found one creating a sigaltstack inside a buffer on a
> pthread stack.  We told everyone to use mmap() and munmap(), with MAP_STACK
> if #ifdef MAP_STACK finds a definition.
>