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

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Date:   Wed, 25 Jan 2023 10:29:20 +0100
From:   David Hildenbrand <david@...hat.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "fweimer@...hat.com" <fweimer@...hat.com>,
        "kees@...nel.org" <kees@...nel.org>
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Subject: Re: [PATCH v5 23/39] mm: Don't allow write GUPs to shadow stack
 memory

On 25.01.23 00:41, Edgecombe, Rick P wrote:
> 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?

I don't think removing that is an option. It's another debug interface 
that has been allowing such access for ever ...

Blocking /proc/self/mem access completely for selected processes might 
be the better alternative.

-- 
Thanks,

David / dhildenb