linux-kernel - Re: [PATCH v5 23/39] mm: Don't allow write GUPs to shadow stack memory

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Message-ID: <19ff6ea3b96d027defb548fb6b7f89de17905a4b.camel@intel.com>
Date:   Tue, 24 Jan 2023 23:41:03 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCH v5 23/39] mm: Don't allow write GUPs to shadow stack
 memory

On Tue, 2023-01-24 at 15:08 -0800, Kees Cook wrote:
> > GDB support for shadow stack is queued up for whenever the kernel
> > interface settles. I believe it just uses ptrace, and not this
> > proc.
> > But yea ptrace poke will still need to use FOLL_FORCE and be able
> > to
> > write through shadow stacks.
> 
> I'd prefer to avoid adding more FOLL_FORCE if we can. If gdb can do
> stack manipulations through a ptrace interface then let's leave off
> FOLL_FORCE.

Ptrace and /proc/self/mem both use FOLL_FORCE. I think ptrace will
always need it or something like it for debugging.

To jog your memory, this series doesn't change what uses FOLL_FORCE. It
just sets the shadow stack rules to be the same as read-only memory. So
even though shadow stack memory is sort of writable, it's a bit more
locked down and FOLL_FORCE is required to write to it with GUP.

If we just remove FOLL_FORCE from /proc/self/mem, something will
probably break right? How do we do this? Some sort of opt-in?