On Mon, Oct 03, 2022 at 07:25:03PM +0200, Jann Horn wrote: > On Mon, Oct 3, 2022 at 7:04 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote: > > On Thu, Sep 29, 2022 at 03:28:57PM -0700, Rick Edgecombe wrote: > > > This is an overdue followup to the “Shadow stacks for userspace” CET series. > > > Thanks for all the comments on the first version [0]. They drove a decent > > > amount of changes for v2. Since it has been awhile, I’ll try to summarize the > > > areas that got major changes since last time. Smaller changes are listed in > > > each patch. > > > > Thanks for the write-up! > > > > > [...] > > > GUP > > > --- > > > Shadow stack memory is generally treated as writable by the kernel, but > > > it behaves differently then other writable memory with respect to GUP. > > > FOLL_WRITE will not GUP shadow stack memory unless FOLL_FORCE is also > > > set. Shadow stack memory is writable from the perspective of being > > > changeable by userspace, but it is also protected memory from > > > userspace’s perspective. So preventing it from being writable via > > > FOLL_WRITE help’s make it harder for userspace to arbitrarily write to > > > it. However, like read-only memory, FOLL_FORCE can still write through > > > it. This means shadow stacks can be written to via things like > > > “/proc/self/mem”. Apps that want extra security will have to prevent > > > access to kernel features that can write with FOLL_FORCE. > > > > This seems like a problem to me -- the point of SS is that there cannot be > > a way to write to them without specific instruction sequences. The fact > > that /proc/self/mem bypasses memory protections was an old design mistake > > that keeps leading to surprising behaviors. It would be much nicer to > > draw the line somewhere and just say that FOLL_FORCE doesn't work on > > VM_SHADOW_STACK. Why must FOLL_FORCE be allowed to write to SS? > > But once you have FOLL_FORCE, you can also just write over stuff like > executable code instead of writing over the stack. I don't think > allowing FOLL_FORCE writes over shadow stacks from /proc/$pid/mem is > making things worse in any way, and it's probably helpful for stuff > like debuggers. > > If you don't want /proc/$pid/mem to be able to do stuff like that, > then IMO the way to go is to change when /proc/$pid/mem uses > FOLL_FORCE, or to limit overall write access to /proc/$pid/mem. Yeah, all reasonable. I just wish we could ditch FOLL_FORCE; it continues to weird me out how powerful that fd's side-effects are. -- Kees Cook
