Re: [RFC] arm64: support HAVE_ARCH_RARE_WRITE

[Hoeun Ryu]

> On Mar 3, 2017, at 1:02 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> 
>> On Thu, Mar 2, 2017 at 7:00 AM, Hoeun Ryu <hoeun.ryu@xxxxxxxxx> 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 (uaccess_enable() replaces the special mm and RW alias is no longer valid.)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@xxxxxxxxx>
> 
> -Kees
> 
> -- 
> Kees Cook
> Pixel Security