From: Mathias Krause <minipli@googlemail.com> To: Thomas Gleixner <tglx@linutronix.de> Cc: Andy Lutomirski <luto@amacapital.net>, Kees Cook <keescook@chromium.org>, Andy Lutomirski <luto@kernel.org>, "kernel-hardening@lists.openwall.com" <kernel-hardening@lists.openwall.com>, Mark Rutland <mark.rutland@arm.com>, Hoeun Ryu <hoeun.ryu@gmail.com>, PaX Team <pageexec@freemail.hu>, Emese Revfy <re.emese@gmail.com>, Russell King <linux@armlinux.org.uk>, X86 ML <x86@kernel.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "linux-arm-kernel@lists.infradead.org" <linux-arm-kernel@lists.infradead.org>, Peter Zijlstra <peterz@infradead.org> Subject: Re: [kernel-hardening] Re: [RFC v2][PATCH 04/11] x86: Implement __arch_rare_write_begin/unmap() Date: Fri, 7 Apr 2017 12:51:15 +0200 [thread overview] Message-ID: <CA+rthh9-EJBBO6nYYY6noP0ybcvk+xJ+S5h6BCNPtidjMCv8VQ@mail.gmail.com> (raw) In-Reply-To: <alpine.DEB.2.20.1704071048360.1716@nanos> On 7 April 2017 at 11:46, Thomas Gleixner <tglx@linutronix.de> wrote: > On Fri, 7 Apr 2017, Mathias Krause wrote: >> On 6 April 2017 at 17:59, Andy Lutomirski <luto@amacapital.net> wrote: >> > On Wed, Apr 5, 2017 at 5:14 PM, Kees Cook <keescook@chromium.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
WARNING: multiple messages have this Message-ID (diff)
From: minipli@googlemail.com (Mathias Krause) To: linux-arm-kernel@lists.infradead.org Subject: [kernel-hardening] Re: [RFC v2][PATCH 04/11] x86: Implement __arch_rare_write_begin/unmap() Date: Fri, 7 Apr 2017 12:51:15 +0200 [thread overview] Message-ID: <CA+rthh9-EJBBO6nYYY6noP0ybcvk+xJ+S5h6BCNPtidjMCv8VQ@mail.gmail.com> (raw) In-Reply-To: <alpine.DEB.2.20.1704071048360.1716@nanos> On 7 April 2017 at 11:46, Thomas Gleixner <tglx@linutronix.de> wrote: > On Fri, 7 Apr 2017, Mathias Krause wrote: >> On 6 April 2017 at 17:59, Andy Lutomirski <luto@amacapital.net> wrote: >> > On Wed, Apr 5, 2017 at 5:14 PM, Kees Cook <keescook@chromium.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
next prev parent reply other threads:[~2017-04-07 10:51 UTC|newest] Thread overview: 188+ messages / expand[flat|nested] mbox.gz Atom feed top 2017-03-29 18:15 [RFC v2] Introduce rare_write() infrastructure Kees Cook 2017-03-29 18:15 ` [kernel-hardening] " Kees Cook 2017-03-29 18:15 ` Kees Cook 2017-03-29 18:15 ` [RFC v2][PATCH 01/11] " Kees Cook 2017-03-29 18:15 ` [kernel-hardening] " Kees Cook 2017-03-29 18:15 ` Kees Cook 2017-03-29 18:23 ` Kees Cook 2017-03-29 18:23 ` [kernel-hardening] " Kees Cook 2017-03-29 18:23 ` Kees Cook 2017-03-30 7:44 ` Ho-Eun Ryu 2017-03-30 7:44 ` [kernel-hardening] " Ho-Eun Ryu 2017-03-30 7:44 ` Ho-Eun Ryu 2017-03-30 17:02 ` Kees Cook 2017-03-30 17:02 ` [kernel-hardening] " Kees Cook 2017-03-30 17:02 ` Kees Cook 2017-04-07 8:09 ` Ho-Eun Ryu 2017-04-07 8:09 ` [kernel-hardening] " Ho-Eun Ryu 2017-04-07 8:09 ` Ho-Eun Ryu 2017-04-07 20:38 ` Kees Cook 2017-04-07 20:38 ` [kernel-hardening] " Kees Cook 2017-04-07 20:38 ` Kees Cook 2017-03-29 18:15 ` [RFC v2][PATCH 02/11] lkdtm: add test for " Kees Cook 2017-03-29 18:15 ` [kernel-hardening] " Kees Cook 2017-03-29 18:15 ` Kees Cook 2017-03-30 9:34 ` [kernel-hardening] " Ian Campbell 2017-03-30 9:34 ` Ian Campbell 2017-03-30 16:16 ` Kees Cook 2017-03-30 16:16 ` Kees Cook 2017-03-30 16:16 ` Kees Cook 2017-03-29 18:15 ` [RFC v2][PATCH 03/11] net: switch sock_diag handlers to rare_write() Kees Cook 2017-03-29 18:15 ` [kernel-hardening] " Kees Cook 2017-03-29 18:15 ` Kees Cook 2017-03-29 18:15 ` [RFC v2][PATCH 04/11] x86: Implement __arch_rare_write_begin/unmap() Kees Cook 2017-03-29 18:15 ` [kernel-hardening] " Kees Cook 2017-03-29 18:15 ` Kees Cook 2017-03-29 22:38 ` Andy Lutomirski 2017-03-29 22:38 ` [kernel-hardening] " Andy Lutomirski 2017-03-29 22:38 ` Andy Lutomirski 2017-03-30 1:41 ` Kees Cook 2017-03-30 1:41 ` [kernel-hardening] " Kees Cook 2017-03-30 1:41 ` Kees Cook 2017-04-05 23:57 ` Andy Lutomirski 2017-04-05 23:57 ` [kernel-hardening] " Andy Lutomirski 2017-04-05 23:57 ` Andy Lutomirski 2017-04-06 0:14 ` Kees Cook 2017-04-06 0:14 ` [kernel-hardening] " Kees Cook 2017-04-06 0:14 ` Kees Cook 2017-04-06 15:59 ` Andy Lutomirski 2017-04-06 15:59 ` [kernel-hardening] " Andy Lutomirski 2017-04-06 15:59 ` Andy Lutomirski 2017-04-07 8:34 ` [kernel-hardening] " Mathias Krause 2017-04-07 8:34 ` Mathias Krause 2017-04-07 8:34 ` Mathias Krause 2017-04-07 9:46 ` Thomas Gleixner 2017-04-07 9:46 ` Thomas Gleixner 2017-04-07 9:46 ` Thomas Gleixner 2017-04-07 10:51 ` Mathias Krause [this message] 2017-04-07 10:51 ` Mathias Krause 2017-04-07 10:51 ` Mathias Krause 2017-04-07 13:14 ` Thomas Gleixner 2017-04-07 13:14 ` Thomas Gleixner 2017-04-07 13:14 ` Thomas Gleixner 2017-04-07 13:30 ` Mathias Krause 2017-04-07 13:30 ` Mathias Krause 2017-04-07 13:30 ` Mathias Krause 2017-04-07 16:14 ` Andy Lutomirski 2017-04-07 16:14 ` Andy Lutomirski 2017-04-07 16:14 ` Andy Lutomirski 2017-04-07 16:22 ` Mark Rutland 2017-04-07 16:22 ` Mark Rutland 2017-04-07 16:22 ` Mark Rutland 2017-04-07 19:58 ` PaX Team 2017-04-07 19:58 ` PaX Team 2017-04-07 19:58 ` PaX Team 2017-04-08 4:58 ` Andy Lutomirski 2017-04-08 4:58 ` Andy Lutomirski 2017-04-08 4:58 ` Andy Lutomirski 2017-04-09 12:47 ` PaX Team 2017-04-09 12:47 ` PaX Team 2017-04-09 12:47 ` PaX Team 2017-04-10 0:10 ` Andy Lutomirski 2017-04-10 0:10 ` Andy Lutomirski 2017-04-10 0:10 ` Andy Lutomirski 2017-04-10 10:42 ` PaX Team 2017-04-10 10:42 ` PaX Team 2017-04-10 10:42 ` PaX Team 2017-04-10 16:01 ` Andy Lutomirski 2017-04-10 16:01 ` Andy Lutomirski 2017-04-10 16:01 ` Andy Lutomirski 2017-04-07 20:44 ` Thomas Gleixner 2017-04-07 20:44 ` Thomas Gleixner 2017-04-07 20:44 ` Thomas Gleixner 2017-04-07 21:20 ` Kees Cook 2017-04-07 21:20 ` Kees Cook 2017-04-07 21:20 ` Kees Cook 2017-04-08 4:12 ` Daniel Micay 2017-04-08 4:12 ` Daniel Micay 2017-04-08 4:12 ` Daniel Micay 2017-04-08 4:13 ` Daniel Micay 2017-04-08 4:13 ` Daniel Micay 2017-04-08 4:13 ` Daniel Micay 2017-04-08 4:21 ` Daniel Micay 2017-04-08 4:21 ` Daniel Micay 2017-04-08 4:21 ` Daniel Micay 2017-04-08 5:07 ` Andy Lutomirski 2017-04-08 5:07 ` Andy Lutomirski 2017-04-08 5:07 ` Andy Lutomirski 2017-04-08 7:33 ` Daniel Micay 2017-04-08 7:33 ` Daniel Micay 2017-04-08 7:33 ` Daniel Micay 2017-04-08 15:20 ` Andy Lutomirski 2017-04-08 15:20 ` Andy Lutomirski 2017-04-08 15:20 ` Andy Lutomirski 2017-04-09 10:53 ` Ingo Molnar 2017-04-09 10:53 ` Ingo Molnar 2017-04-09 10:53 ` Ingo Molnar 2017-04-10 10:22 ` Mark Rutland 2017-04-10 10:22 ` Mark Rutland 2017-04-10 10:22 ` Mark Rutland 2017-04-09 20:24 ` PaX Team 2017-04-09 20:24 ` PaX Team 2017-04-09 20:24 ` PaX Team 2017-04-10 0:31 ` Andy Lutomirski 2017-04-10 0:31 ` Andy Lutomirski 2017-04-10 0:31 ` Andy Lutomirski 2017-04-10 19:47 ` PaX Team 2017-04-10 19:47 ` PaX Team 2017-04-10 19:47 ` PaX Team 2017-04-10 20:27 ` Andy Lutomirski 2017-04-10 20:27 ` Andy Lutomirski 2017-04-10 20:27 ` Andy Lutomirski 2017-04-10 20:13 ` Kees Cook 2017-04-10 20:13 ` Kees Cook 2017-04-10 20:13 ` Kees Cook 2017-04-10 20:17 ` Andy Lutomirski 2017-04-10 20:17 ` Andy Lutomirski 2017-04-10 20:17 ` Andy Lutomirski 2017-04-07 19:25 ` Thomas Gleixner 2017-04-07 19:25 ` Thomas Gleixner 2017-04-07 19:25 ` Thomas Gleixner 2017-04-07 14:45 ` Peter Zijlstra 2017-04-07 14:45 ` Peter Zijlstra 2017-04-07 14:45 ` Peter Zijlstra 2017-04-10 10:29 ` Mark Rutland 2017-04-10 10:29 ` Mark Rutland 2017-04-10 10:29 ` Mark Rutland 2017-04-07 19:52 ` PaX Team 2017-04-07 19:52 ` PaX Team 2017-04-07 19:52 ` PaX Team 2017-04-10 8:26 ` Thomas Gleixner 2017-04-10 8:26 ` Thomas Gleixner 2017-04-10 8:26 ` Thomas Gleixner 2017-04-10 19:55 ` PaX Team 2017-04-10 19:55 ` PaX Team 2017-04-10 19:55 ` PaX Team 2017-04-07 9:37 ` Peter Zijlstra 2017-04-07 9:37 ` [kernel-hardening] " Peter Zijlstra 2017-04-07 9:37 ` Peter Zijlstra 2017-03-29 18:15 ` [RFC v2][PATCH 05/11] ARM: mm: dump: Add domain to output Kees Cook 2017-03-29 18:15 ` [kernel-hardening] " Kees Cook 2017-03-29 18:15 ` Kees Cook 2017-03-29 18:15 ` [RFC v2][PATCH 06/11] ARM: domains: Extract common USER domain init Kees Cook 2017-03-29 18:15 ` [kernel-hardening] " Kees Cook 2017-03-29 18:15 ` Kees Cook 2017-03-29 18:15 ` [RFC v2][PATCH 07/11] ARM: mm: set DOMAIN_WR_RARE for rodata Kees Cook 2017-03-29 18:15 ` [kernel-hardening] " Kees Cook 2017-03-29 18:15 ` Kees Cook 2017-03-29 18:16 ` [RFC v2][PATCH 08/11] ARM: Implement __arch_rare_write_begin/end() Kees Cook 2017-03-29 18:16 ` [kernel-hardening] " Kees Cook 2017-03-29 18:16 ` Kees Cook 2017-04-07 9:36 ` Peter Zijlstra 2017-04-07 9:36 ` [kernel-hardening] " Peter Zijlstra 2017-04-07 9:36 ` Peter Zijlstra 2017-03-29 18:16 ` [RFC v2][PATCH 09/11] list: add rare_write() list helpers Kees Cook 2017-03-29 18:16 ` [kernel-hardening] " Kees Cook 2017-03-29 18:16 ` Kees Cook 2017-03-29 18:16 ` [RFC v2][PATCH 10/11] gcc-plugins: Add constify plugin Kees Cook 2017-03-29 18:16 ` [kernel-hardening] " Kees Cook 2017-03-29 18:16 ` Kees Cook 2017-03-29 18:16 ` [RFC v2][PATCH 11/11] cgroups: force all struct cftype const Kees Cook 2017-03-29 18:16 ` [kernel-hardening] " Kees Cook 2017-03-29 18:16 ` Kees Cook 2017-03-29 19:00 ` [RFC v2] Introduce rare_write() infrastructure Russell King - ARM Linux 2017-03-29 19:00 ` [kernel-hardening] " Russell King - ARM Linux 2017-03-29 19:00 ` Russell King - ARM Linux 2017-03-29 19:14 ` Kees Cook 2017-03-29 19:14 ` [kernel-hardening] " Kees Cook 2017-03-29 19:14 ` Kees Cook
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