linux-kernel - Re: [PATCH v2 02/12] mm: introduce execmem_text_alloc() and jit_text

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Message-Id: <d305b437-9eef-42da-821e-67365aad520b@app.fastmail.com>
Date:   Mon, 17 Jul 2023 10:23:56 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Song Liu" <song@...nel.org>, "Mark Rutland" <mark.rutland@....com>
Cc:     "Mike Rapoport" <rppt@...nel.org>,
        "Kees Cook" <keescook@...omium.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Catalin Marinas" <catalin.marinas@....com>,
        "Christophe Leroy" <christophe.leroy@...roup.eu>,
        "David S. Miller" <davem@...emloft.net>,
        "Dinh Nguyen" <dinguyen@...nel.org>,
        "Heiko Carstens" <hca@...ux.ibm.com>,
        "Helge Deller" <deller@....de>,
        "Huacai Chen" <chenhuacai@...nel.org>,
        "Kent Overstreet" <kent.overstreet@...ux.dev>,
        "Luis Chamberlain" <mcgrof@...nel.org>,
        "Michael Ellerman" <mpe@...erman.id.au>,
        "Nadav Amit" <nadav.amit@...il.com>,
        "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
        "Palmer Dabbelt" <palmer@...belt.com>,
        "Puranjay Mohan" <puranjay12@...il.com>,
        "Rick P Edgecombe" <rick.p.edgecombe@...el.com>,
        "Russell King (Oracle)" <linux@...linux.org.uk>,
        "Steven Rostedt" <rostedt@...dmis.org>,
        "Thomas Bogendoerfer" <tsbogend@...ha.franken.de>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Will Deacon" <will@...nel.org>, bpf@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-mips@...r.kernel.org,
        linux-mm@...ck.org, linux-modules@...r.kernel.org,
        linux-parisc@...r.kernel.org, linux-riscv@...ts.infradead.org,
        linux-s390@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, loongarch@...ts.linux.dev,
        netdev@...r.kernel.org, sparclinux@...r.kernel.org,
        "the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [PATCH v2 02/12] mm: introduce execmem_text_alloc() and jit_text_alloc()



On Mon, Jun 26, 2023, at 10:48 AM, Song Liu wrote:
> On Mon, Jun 26, 2023 at 5:31 AM Mark Rutland <mark.rutland@....com> wrote:
>>
> [...]
>> >
>> > So the idea was that jit_text_alloc() will have a cache of large pages
>> > 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 page will
>> > be "invalidated" by filling it with breakpoint instructions (e.g int3 on
>> > x86)
>>
>> Does that work on x86?
>>
>> That is in no way gauranteed for other architectures; on arm64 you need
>> explicit cache maintenance (with I-cache maintenance at the VA to be executed
>> from) followed by context-synchronization-events (e.g. via ISB instructions, 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 torn 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 rules that might help shed light on this.  I find that this type of request works best when it's really well written :)

>
> Thanks,
> Song