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

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Date:   Tue, 4 Dec 2018 16:03:04 +0000
From:   Will Deacon <will.deacon@....com>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        naveen.n.rao@...ux.vnet.ibm.com, anil.s.keshavamurthy@...el.com,
        David Miller <davem@...emloft.net>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, ast@...nel.org,
        Daniel Borkmann <daniel@...earbox.net>, jeyu@...nel.org,
        netdev@...r.kernel.org, Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Jann Horn <jannh@...gle.com>, kristen@...ux.intel.com,
        Dave Hansen <dave.hansen@...el.com>, deneen.t.dock@...el.com,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages

On Mon, Dec 03, 2018 at 05:43:11PM -0800, Nadav Amit wrote:
> > On Nov 27, 2018, at 4:07 PM, Rick Edgecombe <rick.p.edgecombe@...el.com> wrote:
> > 
> > Since vfree will lazily flush the TLB, but not lazily free the underlying pages,
> > it often leaves stale TLB entries to freed pages that could get re-used. This is
> > undesirable for cases where the memory being freed has special permissions such
> > as executable.
> 
> So I am trying to finish my patch-set for preventing transient W+X mappings
> from taking space, by handling kprobes & ftrace that I missed (thanks again for
> pointing it out).
> 
> But all of the sudden, I don’t understand why we have the problem that this
> (your) patch-set deals with at all. We already change the mappings to make
> the memory writable before freeing the memory, so why can’t we make it
> non-executable at the same time? Actually, why do we make the module memory,
> including its data executable before freeing it???

Yeah, this is really confusing, but I have a suspicion it's a combination
of the various different configurations and hysterical raisins. We can't
rely on module_alloc() allocating from the vmalloc area (see nios2) nor
can we rely on disable_ro_nx() being available at build time.

If we *could* rely on module allocations always using vmalloc(), then
we could pass in Rick's new flag and drop disable_ro_nx() altogether
afaict -- who cares about the memory attributes of a mapping that's about
to disappear anyway?

Is it just nios2 that does something different?

Will