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

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To: Dave Hansen <dave.hansen@xxxxxxxxx>
Subject: Re: [PATCH v2 22/39] mm: Don't allow write GUPs to shadow stack memory
From: Jann Horn <jannh@xxxxxxxxxx>
Date: Sat, 1 Oct 2022 01:02:25 +0200
Cc: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>, x86@xxxxxxxxxx, "H . Peter Anvin" <hpa@xxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, linux-doc@xxxxxxxxxxxxxxx, linux-mm@xxxxxxxxx, linux-arch@xxxxxxxxxxxxxxx, linux-api@xxxxxxxxxxxxxxx, Arnd Bergmann <arnd@xxxxxxxx>, Andy Lutomirski <luto@xxxxxxxxxx>, Balbir Singh <bsingharora@xxxxxxxxx>, Borislav Petkov <bp@xxxxxxxxx>, Cyrill Gorcunov <gorcunov@xxxxxxxxx>, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>, Eugene Syromiatnikov <esyr@xxxxxxxxxx>, Florian Weimer <fweimer@xxxxxxxxxx>, "H . J . Lu" <hjl.tools@xxxxxxxxx>, Jonathan Corbet <corbet@xxxxxxx>, Kees Cook <keescook@xxxxxxxxxxxx>, Mike Kravetz <mike.kravetz@xxxxxxxxxx>, Nadav Amit <nadav.amit@xxxxxxxxx>, Oleg Nesterov <oleg@xxxxxxxxxx>, Pavel Machek <pavel@xxxxxx>, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, Randy Dunlap <rdunlap@xxxxxxxxxxxxx>, "Ravi V . Shankar" <ravi.v.shankar@xxxxxxxxx>, Weijiang Yang <weijiang.yang@xxxxxxxxx>, "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>, joao.moreira@xxxxxxxxx, John Allen <john.allen@xxxxxxx>, kcc@xxxxxxxxxx, eranian@xxxxxxxxxx, rppt@xxxxxxxxxx, jamorris@xxxxxxxxxxxxxxxxxxx, dethoma@xxxxxxxxxxxxx
In-reply-to: <CAG48ez3-dgcrLxKNAs4_K++FXn-9qL=6kjVY=2Cn-AxoML33Vg@mail.gmail.com>
References: <20220929222936.14584-1-rick.p.edgecombe@intel.com> <20220929222936.14584-23-rick.p.edgecombe@intel.com> <9fed0342-2d02-aaf2-ed66-20ff08bdfd0b@intel.com> <CAG48ez3-dgcrLxKNAs4_K++FXn-9qL=6kjVY=2Cn-AxoML33Vg@mail.gmail.com>

On Sat, Oct 1, 2022 at 1:00 AM Jann Horn <jannh@xxxxxxxxxx> wrote:
> So I think this change is actually a no-op - the only thing it does is
> to return 0 if write==1, !_PAGE_RW, and _PAGE_DIRTY. But the check
> below will always return 0 if !_PAGE_RW, unless I'm misreading it?

Er, to be precise, it will always return 0 if write==1 and !_PAGE_RW.

References:
- [PATCH v2 00/39] Shadowstacks for userspace
  - From: Rick Edgecombe
- [PATCH v2 22/39] mm: Don't allow write GUPs to shadow stack memory
  - From: Rick Edgecombe
- Re: [PATCH v2 22/39] mm: Don't allow write GUPs to shadow stack memory
  - From: Dave Hansen
- Re: [PATCH v2 22/39] mm: Don't allow write GUPs to shadow stack memory
  - From: Jann Horn

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