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 16:43:44 +0000
From:   "Schimpe, Christina" <christina.schimpe@...el.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "fweimer@...hat.com" <fweimer@...hat.com>,
        "david@...hat.com" <david@...hat.com>
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        "hpa@...or.com" <hpa@...or.com>,
        "Syromiatnikov, Eugene" <esyr@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
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        "keescook@...omium.org" <keescook@...omium.org>,
        "Eranian, Stephane" <eranian@...gle.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
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        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: RE: [PATCH v5 23/39] mm: Don't allow write GUPs to shadow stack
 memory

> > On Tue, 2023-01-24 at 17:26 +0100, David Hildenbrand wrote:
> > > > > Isn't it possible to overwrite GOT pointers using the same vector?
> > > > > So I think it's merely reflecting the status quo.
> > > >
> > > > There was some debate on this. /proc/self/mem can currently write
> > > > through read-only memory which protects executable code. So should
> > > > shadow stack get separate rules? Is ROP a worry when you can
> > > > overwrite executable code?
> > > >
> > >
> > > The question is, if there is reasonable debugging reason to keep it.
> > > I
> > > assume if a debugger would adjust the ordinary stack, it would have
> > > to adjust the shadow stack as well (oh my ...). So it sounds
> > > reasonable to have it in theory at least ... not sure when debugger
> > > would support that, but maybe they already do.
> >
> > 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.
> 
> Our patches for GDB use /proc/PID/mem to read/write shadow stack
> memory.
> However, I think it should be possible to change this to ptrace but GDB
> normally uses /proc/PID/mem to read/write target memory.
> 
> Regards,
> Christina

I just noticed that GDBSERVER actually uses ptrace, so our patches currently use
both: ptrace and proc/PID/mem to read/write shadow stack memory.

Regards,
Christina
Intel Deutschland GmbH
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