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 . > > 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 . >> >> 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 (uaccess_enable() replaces the special mm and RW alias is no longer valid.)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. >> >>  : http://www.openwall.com/lists/kernel-hardening/2017/02/27/5 >>  : 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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