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

From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
To: Will Deacon <will.deacon@arm.com>
Cc: Andy Lutomirski <luto@kernel.org>,
	Rick Edgecombe <rick.p.edgecombe@intel.com>,
	Nadav Amit <nadav.amit@gmail.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Jessica Yu <jeyu@kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Linux-MM <linux-mm@kvack.org>, Jann Horn <jannh@google.com>,
	"Dock, Deneen T" <deneen.t.dock@intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	kristen@linux.intel.com,
	Andrew Morton <akpm@linux-foundation.org>,
	Ingo Molnar <mingo@redhat.com>,
	anil.s.keshavamurthy@intel.com,
	Kernel Hardening <kernel-hardening@lists.openwall.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	naveen.n.rao@linux.vnet.ibm.com,
	"David S. Miller" <davem@davemloft.net>,
	"<netdev@vger.kernel.org>" <netdev@vger.kernel.org>,
	Dave Hansen <dave.hansen@intel.com>
Subject: Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages
Date: Thu, 6 Dec 2018 20:36:28 +0100	[thread overview]
Message-ID: <CAKv+Gu-NhxUFuce+tZmpiZPLrsNaKWKjtHAg0-6iBnR1VEWc0g@mail.gmail.com> (raw)
In-Reply-To: <20181206193108.GA21002@arm.com>

On Thu, 6 Dec 2018 at 20:30, Will Deacon <will.deacon@arm.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@kernel.org> wrote:
> > >
> > > On Thu, Dec 6, 2018 at 11:04 AM Ard Biesheuvel
> > > <ard.biesheuvel@linaro.org> wrote:
> > > >
> > > > On Thu, 6 Dec 2018 at 19:54, Andy Lutomirski <luto@kernel.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.