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

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Message-Id: <08141F66-F3E6-4CC5-AF91-1ED5F101A54C@gmail.com>
Date:   Tue, 4 Dec 2018 11:44:58 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Will Deacon <will.deacon@....com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        "Naveen N . Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, jeyu@...nel.org,
        Network Development <netdev@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Jann Horn <jannh@...gle.com>,
        Kristen Carlson Accardi <kristen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        "Dock, Deneen T" <deneen.t.dock@...el.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages

> On Dec 4, 2018, at 10:56 AM, Andy Lutomirski <luto@...nel.org> wrote:
> 
> On Mon, Dec 3, 2018 at 5:43 PM Nadav Amit <nadav.amit@...il.com> 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???
> 
> All the code you're looking at is IMO a very awkward and possibly
> incorrect of doing what's actually necessary: putting the direct map
> the way it wants to be.
> 
> Can't we shove this entirely mess into vunmap?  Have a flag (as part
> of vmalloc like in Rick's patch or as a flag passed to a vfree variant
> directly) that makes the vunmap code that frees the underlying pages
> also reset their permissions?
> 
> Right now, we muck with set_memory_rw() and set_memory_nx(), which
> both have very awkward (and inconsistent with each other!) semantics
> when called on vmalloc memory.  And they have their own flushes, which
> is inefficient.  Maybe the right solution is for vunmap to remove the
> vmap area PTEs, call into a function like set_memory_rw() that resets
> the direct maps to their default permissions *without* flushing, and
> then to do a single flush for everything.  Or, even better, to cause
> the change_page_attr code to do the flush and also to flush the vmap
> area all at once so that very small free operations can flush single
> pages instead of flushing globally.

Thanks for the explanation. I read it just after I realized that indeed the
whole purpose of this code is to get cpa_process_alias() 
update the corresponding direct mapping.

This thing (pageattr.c) indeed seems over-engineered and very unintuitive.
Right now I have a list of patch-sets that I owe, so I don’t have the time
to deal with it.

But, I still think that disable_ro_nx() should not call set_memory_x().
IIUC, this breaks W+X of the direct-mapping which correspond with the module
memory. Does it ever stop being W+X?? I’ll have another look.