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Message-Id: <E00348D0-114B-4803-B80E-0500BF3D96D7@gmail.com>
Date: Sat, 4 Mar 2017 14:53:20 +0900
From: Hoeun Ryu <hoeun.ryu@...il.com>
To: Kees Cook <keescook@...omium.org>
Cc: "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
LKML <linux-kernel@...r.kernel.org>, Mark Rutland <mark.rutland@....com>,
Andy Lutomirski <luto@...nel.org>, Emese Revfy <re.emese@...il.com>,
Russell King <linux@...linux.org.uk>, PaX Team <pageexec@...email.hu>,
"x86@...nel.org" <x86@...nel.org>
Subject: Re: [RFC] arm64: support HAVE_ARCH_RARE_WRITE
> On Mar 3, 2017, at 1:02 PM, Kees Cook <keescook@...omium.org> wrote:
>
>> On Thu, Mar 2, 2017 at 7:00 AM, Hoeun Ryu <hoeun.ryu@...il.com> wrote:
>> This RFC is a quick and dirty arm64 implementation for Kees Cook's RFC for
>> rare_write infrastructure [1].
>
> Awesome! :)
>
>> This implementation is based on Mark Rutland's suggestions, which is that
>> a special userspace mm that maps only __start/end_rodata as RW permission
>> is prepared during early boot time (paging_init) and __arch_rare_write_map()
>> switches to the mm [2].
>>
>> Due to the limit of implementation (the mm having RW mapping is userspace
>> mm), we need a new arch-specific __arch_rare_write_ptr() to convert RO
>> address to RW address (CONFIG_HAVE_RARE_WRITE_PTR is added), which is
>> general for all architectures (__rare_write_ptr()) in Kees's RFC . So all
>> writes should be instrumented by __rare_write().
>
> Cool, yeah, I'll get all this fixed up in my next version.
I'll send the next version of this when you send yours.
>> One caveat for arm64 is CONFIG_ARM64_SW_TTBR0_PAN.
>> Because __arch_rare_write_map() installes a special user mm to ttbr0,
>> usercopy inside __arch_rare_write_map/unmap() pair will break rare_write.
>> (uaccess_enable() replaces the special mm and RW alias is no longer valid.)
>
> That's totally fine constraint: this case should never happen for so
> many reasons. :)
>
OK. Thank you for the review.
>> A similar problem could rise in general usercopy inside
>> __arch_rare_write_map/unmap(). __arch_rare_write_map() replaces current->mm,
>> so we loose the address space of the `current` process.
>>
>> It passes LKDTM's rare write test.
>>
>> [1] : http://www.openwall.com/lists/kernel-hardening/2017/02/27/5
>> [2] : https://lkml.org/lkml/2017/2/22/254
>>
>> Signed-off-by: Hoeun Ryu <hoeun.ryu@...il.com>
>
> -Kees
>
> --
> Kees Cook
> Pixel Security
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