linux-kernel - Re: [PATCH bpf-next] bpf: Make trampolines W^X

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Message-Id: <DB882EE8-20B2-4631-A808-E5C968B24CEB@amacapital.net>
Date:   Mon, 6 Jan 2020 15:36:49 -1000
From:   Andy Lutomirski <luto@...capital.net>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc:     "kpsingh@...omium.org" <kpsingh@...omium.org>,
        "songliubraving@...com" <songliubraving@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "kuznet@....inr.ac.ru" <kuznet@....inr.ac.ru>,
        "jannh@...gle.com" <jannh@...gle.com>,
        "mjg59@...gle.com" <mjg59@...gle.com>,
        "thgarnie@...omium.org" <thgarnie@...omium.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "revest@...omium.org" <revest@...omium.org>,
        "jackmanb@...omium.org" <jackmanb@...omium.org>,
        "kafai@...com" <kafai@...com>, "yhs@...com" <yhs@...com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "yoshfuji@...ux-ipv6.org" <yoshfuji@...ux-ipv6.org>,
        "mhalcrow@...gle.com" <mhalcrow@...gle.com>,
        "andriin@...com" <andriin@...com>
Subject: Re: [PATCH bpf-next] bpf: Make trampolines W^X


> On Jan 6, 2020, at 12:25 PM, Edgecombe, Rick P <rick.p.edgecombe@...el.com> wrote:
> 
> On Sat, 2020-01-04 at 09:49 +0900, Andy Lutomirski wrote:
>>>> On Jan 4, 2020, at 8:47 AM, KP Singh <kpsingh@...omium.org> wrote:
>>> 
>>> From: KP Singh <kpsingh@...gle.com>
>>> 
>>> The image for the BPF trampolines is allocated with
>>> bpf_jit_alloc_exe_page which marks this allocated page executable. This
>>> means that the allocated memory is W and X at the same time making it
>>> susceptible to WX based attacks.
>>> 
>>> Since the allocated memory is shared between two trampolines (the
>>> current and the next), 2 pages must be allocated to adhere to W^X and
>>> the following sequence is obeyed where trampolines are modified:
>> 
>> Can we please do better rather than piling garbage on top of garbage?
>> 
>>> 
>>> - Mark memory as non executable (set_memory_nx). While module_alloc for
>>> x86 allocates the memory as PAGE_KERNEL and not PAGE_KERNEL_EXEC, not
>>> all implementations of module_alloc do so
>> 
>> How about fixing this instead?
>> 
>>> - Mark the memory as read/write (set_memory_rw)
>> 
>> Probably harmless, but see above about fixing it.
>> 
>>> - Modify the trampoline
>> 
>> Seems reasonable. It’s worth noting that this whole approach is suboptimal:
>> the “module” allocator should really be returning a list of pages to be
>> written (not at the final address!) with the actual executable mapping to be
>> materialized later, but that’s a bigger project that you’re welcome to ignore
>> for now.  (Concretely, it should produce a vmap address with backing pages but
>> with the vmap alias either entirely unmapped or read-only. A subsequent healer
>> would, all at once, make the direct map pages RO or not-present and make the
>> vmap alias RX.)
>>> - Mark the memory as read-only (set_memory_ro)
>>> - Mark the memory as executable (set_memory_x)
>> 
>> No, thanks. There’s very little excuse for doing two IPI flushes when one
>> would suffice.
>> 
>> As far as I know, all architectures can do this with a single flush without
>> races  x86 certainly can. The module freeing code gets this sequence right.
>> Please reuse its mechanism or, if needed, export the relevant interfaces.
> 
> So if I understand this right, some trampolines have been added that are
> currently set as RWX at modification time AND left that way during runtime? The
> discussion on the order of set_memory_() calls in the commit message made me
> think that this was just a modification time thing at first.

I’m not sure what the status quo is.

We really ought to have a genuinely good API for allocation and initialization of text.  We can do so much better than set_memory_blahblah.

FWIW, I have some ideas about making kernel flushes cheaper. It’s currently blocked on finding some time and on tglx’s irqtrace work.

> 
> Also, is there a reason you couldn't use text_poke() to modify the trampoline
> with a single flush?
> 

Does text_poke to an IPI these days?