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Message-Id: <32219CAE-7D49-4848-9497-A17E0D809B3E@gmail.com>
Date: Thu, 17 Jan 2019 14:29:36 -0800
From: Nadav Amit <nadav.amit@...il.com>
To: Andy Lutomirski <luto@...nel.org>
Cc: Rick Edgecombe <rick.p.edgecombe@...el.com>,
Ingo Molnar <mingo@...hat.com>,
LKML <linux-kernel@...r.kernel.org>,
X86 ML <x86@...nel.org>,
"H. Peter Anvin" <hpa@...or.com>,
Thomas Gleixner <tglx@...utronix.de>,
Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Peter Zijlstra <peterz@...radead.org>,
Damian Tometzki <linux_dti@...oud.com>,
linux-integrity <linux-integrity@...r.kernel.org>,
LSM List <linux-security-module@...r.kernel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
Linux-MM <linux-mm@...ck.org>,
Will Deacon <will.deacon@....com>,
Ard Biesheuvel <ard.biesheuvel@...aro.org>,
Kristen Carlson Accardi <kristen@...ux.intel.com>,
"Dock, Deneen T" <deneen.t.dock@...el.com>,
Kees Cook <keescook@...omium.org>,
Dave Hansen <dave.hansen@...el.com>,
Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH 06/17] x86/alternative: use temporary mm for text poking
> On Jan 17, 2019, at 1:43 PM, Nadav Amit <nadav.amit@...il.com> wrote:
>
>> On Jan 17, 2019, at 12:47 PM, Andy Lutomirski <luto@...nel.org> wrote:
>>
>> On Thu, Jan 17, 2019 at 12:27 PM Andy Lutomirski <luto@...nel.org> wrote:
>>> On Wed, Jan 16, 2019 at 4:33 PM Rick Edgecombe
>>> <rick.p.edgecombe@...el.com> wrote:
>>>> From: Nadav Amit <namit@...are.com>
>>>>
>>>> text_poke() can potentially compromise the security as it sets temporary
>>>> PTEs in the fixmap. These PTEs might be used to rewrite the kernel code
>>>> from other cores accidentally or maliciously, if an attacker gains the
>>>> ability to write onto kernel memory.
>>>
>>> i think this may be sufficient, but barely.
>>>
>>>> + pte_clear(poking_mm, poking_addr, ptep);
>>>> +
>>>> + /*
>>>> + * __flush_tlb_one_user() performs a redundant TLB flush when PTI is on,
>>>> + * as it also flushes the corresponding "user" address spaces, which
>>>> + * does not exist.
>>>> + *
>>>> + * Poking, however, is already very inefficient since it does not try to
>>>> + * batch updates, so we ignore this problem for the time being.
>>>> + *
>>>> + * Since the PTEs do not exist in other kernel address-spaces, we do
>>>> + * not use __flush_tlb_one_kernel(), which when PTI is on would cause
>>>> + * more unwarranted TLB flushes.
>>>> + *
>>>> + * There is a slight anomaly here: the PTE is a supervisor-only and
>>>> + * (potentially) global and we use __flush_tlb_one_user() but this
>>>> + * should be fine.
>>>> + */
>>>> + __flush_tlb_one_user(poking_addr);
>>>> + if (cross_page_boundary) {
>>>> + pte_clear(poking_mm, poking_addr + PAGE_SIZE, ptep + 1);
>>>> + __flush_tlb_one_user(poking_addr + PAGE_SIZE);
>>>> + }
>>>
>>> In principle, another CPU could still have the old translation. Your
>>> mutex probably makes this impossible, but it makes me nervous.
>>> Ideally you'd use flush_tlb_mm_range(), but I guess you can't do that
>>> with IRQs off. Hmm. I think you should add an inc_mm_tlb_gen() here.
>>> Arguably, if you did that, you could omit the flushes, but maybe
>>> that's silly.
>>>
>>> If we start getting new users of use_temporary_mm(), we should give
>>> some serious thought to the SMP semantics.
>>>
>>> Also, you're using PAGE_KERNEL. Please tell me that the global bit
>>> isn't set in there.
>>
>> Much better solution: do unuse_temporary_mm() and *then*
>> flush_tlb_mm_range(). This is entirely non-sketchy and should be just
>> about optimal, too.
>
> This solution sounds nice and clean. The fact the global-bit was set didn’t
> matter before (since __flush_tlb_one_user would get rid of it no matter
> what), but would matter now, so I’ll change it too.
Err.. so actually text_poke() might be called with disabled IRQs (by kgdb).
flush_tlb_mm_range() should still work fine even with disabled IRQs since no
core would use poking_mm at this point. I can add a comment to
flush_tlb_mm_range(), but all in all it is actually not very pretty.
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