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Date: Mon, 4 Jun 2018 17:13:27 +0800
From: Jun Yao <yaojun8558363@...il.com>
To: robin.murphy@....com, linux-arm-kernel@...ts.infradead.org
Cc: catalin.marinas@....com, will.deacon@....com,
	linux-kernel@...r.kernel.org, greg@...ah.com,
	kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH 0/4] arm64/mm: migrate swapper_pg_dir

On 1 Jun 2018 at 10:42:10, Robin Murphy <robin.murphy@....com> wrote:
>> Currently, The offset between swapper_pg_dir and _text is
>> fixed. When attackers know the address of _text(no KASLR or
>> breaking KASLR), they can caculate the address of
>> swapper_pg_dir. Then KSMA(Kernel Space Mirroring Attack) can
>> be applied.
>>
>> The principle of KSMA is to insert a carefully constructed PGD
>> entry into the translation table. The type of this entry is
>
>Out of interest, how does that part work? AFAICS, modifying a PGD entry
>involves writing to kernel memory, which would mean the implication of KSMA is
>"userspace can gain write permission to kernel memory by writing to kernel
>memory" - that doesn't sound like an attack in itself, more just a convenience
>for ease of exploiting whatever successful attack got you in there in the
>first place.
>
>That's not to say that it isn't still worth mitigating, I'm just questioning
>the given rationale here.

Yes, you are right. KSMA is just a convenience for ease of exploiting. I think
that the biggest role of KSMA is to covert an arbitrary write to multiple
arbitrary writes. In the past, to accomplish this, a function
pointer(e.g. ptmx_fops) is modified to point to gadget, which can r/w kernel
memory. However, PAN makes this more difficult. And KSMA becomes a new way to
do that.

For details on KSMA, you can refer to:

https://www.blackhat.com/docs/asia-18/asia-18-WANG-KSMA-Breaking-Android-kernel-isolation-and-Rooting-with-ARM-MMU-features.pdf

thanks,

Jun

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