netdev - Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages

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Date:   Thu, 6 Dec 2018 20:36:28 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Will Deacon <will.deacon@....com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jessica Yu <jeyu@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Linux-MM <linux-mm@...ck.org>, Jann Horn <jannh@...gle.com>,
        "Dock, Deneen T" <deneen.t.dock@...el.com>,
        Peter Zijlstra <peterz@...radead.org>, kristen@...ux.intel.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...hat.com>, anil.s.keshavamurthy@...el.com,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        naveen.n.rao@...ux.vnet.ibm.com,
        "David S. Miller" <davem@...emloft.net>,
        "<netdev@...r.kernel.org>" <netdev@...r.kernel.org>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages

On Thu, 6 Dec 2018 at 20:30, Will Deacon <will.deacon@....com> wrote:
>
> On Thu, Dec 06, 2018 at 08:23:20PM +0100, Ard Biesheuvel wrote:
> > On Thu, 6 Dec 2018 at 20:21, Andy Lutomirski <luto@...nel.org> wrote:
> > >
> > > On Thu, Dec 6, 2018 at 11:04 AM Ard Biesheuvel
> > > <ard.biesheuvel@...aro.org> wrote:
> > > >
> > > > On Thu, 6 Dec 2018 at 19:54, Andy Lutomirski <luto@...nel.org> wrote:
> > > > >
> > >
> > > > > That’s not totally nuts. Do we ever have code that expects __va() to
> > > > > work on module data?  Perhaps crypto code trying to encrypt static
> > > > > data because our APIs don’t understand virtual addresses.  I guess if
> > > > > highmem is ever used for modules, then we should be fine.
> > > > >
> > > >
> > > > The crypto code shouldn't care, but I think it will probably break hibernate :-(
> > >
> > > How so?  Hibernate works (or at least should work) on x86 PAE, where
> > > __va doesn't work on module data, and, on x86, the direct map has some
> > > RO parts with where the module is, so hibernate can't be writing to
> > > the memory through the direct map with its final permissions.
> >
> > On arm64 at least, hibernate reads the contents of memory via the
> > linear mapping. Not sure about other arches.
>
> Can we handle this like the DEBUG_PAGEALLOC case, and extract the pfn from
> the pte when we see that it's PROT_NONE?
>

As long as we can easily figure out whether a certain linear address
is mapped or not, having a special case like that for these mappings
doesn't sound unreasonable.