Re: [PATCH v2 22/39] mm: Don't allow write GUPs to shadow stack memory

[Andy Lutomirski]

On 10/3/22 11:39, Kees Cook wrote:
> On Thu, Sep 29, 2022 at 03:29:19PM -0700, Rick Edgecombe wrote:
> > [...]
> > Still allow FOLL_FORCE to write through shadow stack protections, as it
> > does for read-only protections.
>
> As I asked in the cover letter: why do we need to add this for shstk? It
> was a mistake for general memory. :P

For debuggers, which use FOLL_FORCE, quite intentionally, to modify 
text.  And once a debugger has ptrace write access to a target, shadow 
stacks provide exactly no protection -- ptrace can modify text and all 
registers.

But /proc/.../mem may be a different story, and I'd be okay with having 
FOLL_PROC_MEM for legacy compatibility via /proc/.../mem and not 
allowing that to access shadow stacks.  This does seem like it may not 
be very useful, though.