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Date:   Mon, 17 Jul 2023 10:23:56 -0700
From:   "Andy Lutomirski" <luto@kernel.org>
To:     "Song Liu" <song@kernel.org>, "Mark Rutland" <mark.rutland@arm.com>
Cc:     "Mike Rapoport" <rppt@kernel.org>,
        "Kees Cook" <keescook@chromium.org>,
        "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
        "Andrew Morton" <akpm@linux-foundation.org>,
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        "Christophe Leroy" <christophe.leroy@csgroup.eu>,
        "David S. Miller" <davem@davemloft.net>,
        "Dinh Nguyen" <dinguyen@kernel.org>,
        "Heiko Carstens" <hca@linux.ibm.com>,
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        "Huacai Chen" <chenhuacai@kernel.org>,
        "Kent Overstreet" <kent.overstreet@linux.dev>,
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Subject: Re: [PATCH v2 02/12] mm: introduce execmem_text_alloc() and jit_text_alloc()
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On Mon, Jun 26, 2023, at 10:48 AM, Song Liu wrote:
> On Mon, Jun 26, 2023 at 5:31=E2=80=AFAM Mark Rutland <mark.rutland@arm=
.com> wrote:
>>
> [...]
>> >
>> > So the idea was that jit_text_alloc() will have a cache of large pa=
ges
>> > mapped ROX, will allocate memory from those caches and there will be
>> > jit_update() that uses text poking for writing to that memory.
>> >
>> > Upon allocation of a large page to increase the cache, that large p=
age will
>> > be "invalidated" by filling it with breakpoint instructions (e.g in=
t3 on
>> > x86)
>>
>> Does that work on x86?
>>
>> That is in no way gauranteed for other architectures; on arm64 you ne=
ed
>> explicit cache maintenance (with I-cache maintenance at the VA to be =
executed
>> from) followed by context-synchronization-events (e.g. via ISB instru=
ctions, or
>> IPIs).
>
> I guess we need:
> 1) Invalidate unused part of the huge ROX pages;
> 2) Do not put two jit users (including module text, bpf, etc.) in the
> same cache line;
> 3) Explicit cache maintenance;
> 4) context-synchronization-events.
>
> Would these (or a subset of them) be sufficient to protect us from tor=
n read?

Maybe?  #4 is sufficiently vague that I can't really interpret it.

I have a half-drafted email asking for official clarification on the rul=
es that might help shed light on this.  I find that this type of request=
 works best when it's really well written :)

>
> Thanks,
> Song