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Subject: Re: TLB flushes on fixmap changes
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <CAGXu5j+xUbq_mu=2jvH2Vu+mviteZJqdPNTrxpaijwsuDdN-sw@mail.gmail.com>
Date:   Sat, 25 Aug 2018 22:53:47 -0700
Cc:     Masami Hiramatsu <mhiramat@kernel.org>,
        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>,
        Andy Lutomirski <luto@kernel.org>
Sender: linux-kernel-owner@vger.kernel.org
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at 9:43 PM, Kees Cook <keescook@chromium.org> wrote:

> On Sat, Aug 25, 2018 at 9:21 PM, Andy Lutomirski <luto@kernel.org> =
wrote:
>> 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 =
ioremap_cache?
>>=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=3D=
kspp/write-rarely&id=3D9ab0cb2618ebbc51f830ceaa06b7d2182fe1a52d
>=20
> With that, text_poke() mostly becomes:
>=20
> rare_write_begin()
> memcpy(addr, opcode, len);
> rare_write_end()
>=20
> As for juiciness, if an attacker already has execution control, they
> can do more interesting things than text_poke(). But regardless, yes,
> it's always made me uncomfortable. :)

I think that the key to harden the security of text_poke() against its =
use
as a gadget in a ROP/JOP attack is to add a check/assertion for the old
(expected) value, such as:

rare_write_begin()
if (*addr =3D=3D prev_opcode)
	memcpy(addr, opcode, len);
rare_write_end()