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

[Ard Biesheuvel]

On Thu, 6 Dec 2018 at 20:30, Will Deacon <will.deacon@xxxxxxx> 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@xxxxxxxxxx> wrote:
> > >
> > > On Thu, Dec 6, 2018 at 11:04 AM Ard Biesheuvel
> > > <ard.biesheuvel@xxxxxxxxxx> wrote:
> > > >
> > > > On Thu, 6 Dec 2018 at 19:54, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> > > > >
> > >
> > > > > Thatâs 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 donât 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.