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Subject: Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <CALCETrXvddt148fncMJqpjK98uatiK-44knYFWU0-ytf8X+iog@mail.gmail.com>
Date:   Tue, 4 Dec 2018 11:44:58 -0800
Cc:     Rick Edgecombe <rick.p.edgecombe@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Will Deacon <will.deacon@arm.com>,
        Linux-MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        "Naveen N . Rao" <naveen.n.rao@linux.vnet.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>,
        "David S. Miller" <davem@davemloft.net>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>, jeyu@kernel.org,
        Network Development <netdev@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Jann Horn <jannh@google.com>,
        Kristen Carlson Accardi <kristen@linux.intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        "Dock, Deneen T" <deneen.t.dock@intel.com>,
        Peter Zijlstra <peterz@infradead.org>
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References: <20181128000754.18056-1-rick.p.edgecombe@intel.com>
 <20181128000754.18056-2-rick.p.edgecombe@intel.com>
 <4883FED1-D0EC-41B0-A90F-1A697756D41D@gmail.com>
 <CALCETrXvddt148fncMJqpjK98uatiK-44knYFWU0-ytf8X+iog@mail.gmail.com>
To:     Andy Lutomirski <luto@kernel.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Dec 4, 2018, at 10:56 AM, Andy Lutomirski <luto@kernel.org> wrote:
>=20
> On Mon, Dec 3, 2018 at 5:43 PM Nadav Amit <nadav.amit@gmail.com> =
wrote:
>>> On Nov 27, 2018, at 4:07 PM, Rick Edgecombe =
<rick.p.edgecombe@intel.com> wrote:
>>>=20
>>> 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.
>>=20
>> 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).
>>=20
>> But all of the sudden, I don=E2=80=99t 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=E2=80=99t =
we make it
>> non-executable at the same time? Actually, why do we make the module =
memory,
>> including its data executable before freeing it???
>=20
> 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.
>=20
> 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?
>=20
> 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()=20
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=E2=80=99t =
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=E2=80=99ll have another look.