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Subject: Re: TLB flushes on fixmap changes
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <CAGXu5jKk+ELGsSXC8e3v67oo74BF9rP2HDqMHx1Sb17-0F-xZQ@mail.gmail.com>
Date:   Sun, 26 Aug 2018 10:25:11 -0700
Cc:     Andy Lutomirski <luto@kernel.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Nadav Amit <nadav.amit@gmail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Jiri Kosina <jkosina@suse.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will.deacon@arm.com>,
        Benjamin Herrenschmidt <benh@au1.ibm.com>,
        Nick Piggin <npiggin@gmail.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Rik van Riel <riel@surriel.com>,
        Jann Horn <jannh@google.com>,
        Adin Scannell <ascannell@google.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>,
        David Miller <davem@davemloft.net>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Michael Ellerman <mpe@ellerman.id.au>
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To:     Kees Cook <keescook@chromium.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Aug 26, 2018, at 9:47 AM, Kees Cook <keescook@chromium.org> wrote:
>=20
>> On Sun, Aug 26, 2018 at 7:20 AM, Andy Lutomirski <luto@amacapital.net> wr=
ote:
>>=20
>>=20
>>>> On Aug 25, 2018, at 9:43 PM, Kees Cook <keescook@chromium.org> wrote:
>>>>=20
>>>>> On Sat, Aug 25, 2018 at 9:21 PM, Andy Lutomirski <luto@kernel.org> wro=
te:
>>>>> On Sat, Aug 25, 2018 at 7:23 PM, Masami Hiramatsu <mhiramat@kernel.org=
> wrote:
>>>>> On Fri, 24 Aug 2018 21:23:26 -0700
>>>>> Andy Lutomirski <luto@kernel.org> wrote:
>>>>>> Couldn't text_poke() use kmap_atomic()?  Or, even better, just change=
 CR3?
>>>>>=20
>>>>> No, since kmap_atomic() is only for x86_32 and highmem support kernel.=

>>>>> In x86-64, it seems that returns just a page address. That is not
>>>>> good for text_poke, since it needs to make a writable alias for RO
>>>>> code page. Hmm, maybe, can we mimic copy_oldmem_page(), it uses iorema=
p_cache?
>>>>>=20
>>>>=20
>>>> I just re-read text_poke().  It's, um, horrible.  Not only is the
>>>> implementation overcomplicated and probably buggy, but it's SLOOOOOW.
>>>> It's totally the wrong API -- poking one instruction at a time
>>>> basically can't be efficient on x86.  The API should either poke lots
>>>> of instructions at once or should be text_poke_begin(); ...;
>>>> text_poke_end();.
>>>>=20
>>>> Anyway, the attached patch seems to boot.  Linus, Kees, etc: is this
>>>> too scary of an approach?  With the patch applied, text_poke() is a
>>>> fantastic exploit target.  On the other hand, even without the patch
>>>> applied, text_poke() is every bit as juicy.
>>>=20
>>> I tried to convince Ingo to use this method for doing "write rarely"
>>> and he soundly rejected it. :) I've always liked this because AFAICT,
>>> it's local to the CPU. I had proposed it in
>>> https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=
=3Dkspp/write-rarely&id=3D9ab0cb2618ebbc51f830ceaa06b7d2182fe1a52d
>>=20
>> Ingo, can you clarify why you hate it?  I personally would rather use CR3=
, but CR0 seems like a fine first step, at least for text_poke.
>=20
> Sorry, it looks like it was tglx, not Ingo:
>=20
> https://lkml.kernel.org/r/alpine.DEB.2.20.1704071048360.1716@nanos
>=20
> This thread is long, and one thing that I think went unanswered was
> "why do we want this to be fast?" the answer is: for doing page table
> updates. Page tables are becoming a bigger target for attacks now, and
> it's be nice if they could stay read-only unless they're getting
> updated (with something like this).
>=20
>=20

It kind of sounds like tglx would prefer the CR3 approach. And indeed my pat=
ch has a serious problem wrt the NMI code.