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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: <CALCETrXLrsKDBzDkN7sc9HYPWe9aV3NQzf4vMvM+FD8j6aA6AQ@mail.gmail.com>
Date:   Tue, 4 Dec 2018 14:48:40 -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>
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 <4883FED1-D0EC-41B0-A90F-1A697756D41D@gmail.com>
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To:     Andy Lutomirski <luto@kernel.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Dec 4, 2018, at 11:48 AM, Andy Lutomirski <luto@kernel.org> wrote:
>=20
> On Tue, Dec 4, 2018 at 11:45 AM Nadav Amit <nadav.amit@gmail.com> =
wrote:
>>> 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.
>>=20
>> 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.
>>=20
>> 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.
>>=20
>> 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.
>=20
> Dunno.  I did once chase down a bug where some memory got freed while
> it was still read-only, and the results were hilarious and hard to
> debug, since the explosion happened long after the buggy code
> finished.

This piece of code causes me pain and misery.

So, it turns out that the direct map is *not* changed if you just change
the NX-bit. See change_page_attr_set_clr():

        /* No alias checking for _NX bit modifications */
        checkalias =3D (pgprot_val(mask_set) | pgprot_val(mask_clr)) !=3D =
_PAGE_NX;

How many levels of abstraction are broken in the way? What would happen
if somebody tries to change the NX-bit and some other bit in the PTE?
Luckily, I don=E2=80=99t think someone does=E2=80=A6 at least for now.

So, again, I think the change I proposed makes sense. nios2 does not =
have
set_memory_x() and it will not be affected.

[ I can add a comment, although I don=E2=80=99t have know if nios2 has =
an NX bit,
and I don=E2=80=99t find any code that defines PTEs. Actually where is =
pte_present()
of nios2 being defined? Whatever. ]