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Message-ID: <CA+rthh9-EJBBO6nYYY6noP0ybcvk+xJ+S5h6BCNPtidjMCv8VQ@mail.gmail.com> Date: Fri, 7 Apr 2017 12:51:15 +0200 From: Mathias Krause <minipli@...glemail.com> To: Thomas Gleixner <tglx@...utronix.de> Cc: Andy Lutomirski <luto@...capital.net>, Kees Cook <keescook@...omium.org>, Andy Lutomirski <luto@...nel.org>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, Mark Rutland <mark.rutland@....com>, Hoeun Ryu <hoeun.ryu@...il.com>, PaX Team <pageexec@...email.hu>, Emese Revfy <re.emese@...il.com>, Russell King <linux@...linux.org.uk>, X86 ML <x86@...nel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, Peter Zijlstra <peterz@...radead.org> Subject: Re: Re: [RFC v2][PATCH 04/11] x86: Implement __arch_rare_write_begin/unmap() On 7 April 2017 at 11:46, Thomas Gleixner <tglx@...utronix.de> wrote: > On Fri, 7 Apr 2017, Mathias Krause wrote: >> On 6 April 2017 at 17:59, Andy Lutomirski <luto@...capital.net> wrote: >> > On Wed, Apr 5, 2017 at 5:14 PM, Kees Cook <keescook@...omium.org> wrote: >> >> static __always_inline rare_write_begin(void) >> >> { >> >> preempt_disable(); >> >> local_irq_disable(); >> >> barrier(); >> >> __arch_rare_write_begin(); >> >> barrier(); >> >> } >> > >> > Looks good, except you don't need preempt_disable(). >> > local_irq_disable() also disables preemption. You might need to use >> > local_irq_save(), though, depending on whether any callers already >> > have IRQs off. >> >> Well, doesn't look good to me. NMIs will still be able to interrupt >> this code and will run with CR0.WP = 0. >> >> Shouldn't you instead question yourself why PaX can do it "just" with >> preempt_disable() instead?! > > That's silly. Just because PaX does it, doesn't mean it's correct. To be > honest, playing games with the CR0.WP bit is outright stupid to begin with. Why that? It allows fast and CPU local modifications of r/o memory. OTOH, an approach that needs to fiddle with page table entries requires global synchronization to keep the individual TLB states in sync. Hmm.. Not that fast, I'd say. > Whether protected by preempt_disable or local_irq_disable, to make that > work it needs CR0 handling in the exception entry/exit at the lowest > level. And that's just a nightmare maintainence wise as it's prone to be > broken over time. It seems to be working fine for more than a decade now in PaX. So it can't be such a big maintenance nightmare ;) > Aside of that it's pointless overhead for the normal case. > > The proper solution is: > > write_rare(ptr, val) > { > mp = map_shadow_rw(ptr); > *mp = val; > unmap_shadow_rw(mp); > } > > map_shadow_rw() is essentially the same thing as we do in the highmem case > where the kernel creates a shadow mapping of the user space pages via > kmap_atomic(). The "proper solution" seems to be much slower compared to just toggling CR0.WP (which is costly in itself, already) because of the TLB invalidation / synchronisation involved. > It's valid (at least on x86) to have a shadow map with the same page > attributes but write enabled. That does not require any fixups of CR0 and > just works. "Just works", sure -- but it's not as tightly focused as the PaX solution which is CPU local, while your proposed solution is globally visible. Cheers, Mathias
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