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Message-ID: <58E7EF70.30766.621C4F44@pageexec.freemail.hu> Date: Fri, 07 Apr 2017 21:58:40 +0200 From: "PaX Team" <pageexec@...email.hu> To: Mathias Krause <minipli@...glemail.com>, Andy Lutomirski <luto@...nel.org> CC: Thomas Gleixner <tglx@...utronix.de>, 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>, 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 Apr 2017 at 9:14, Andy Lutomirski wrote: > On Fri, Apr 7, 2017 at 6:30 AM, Mathias Krause <minipli@...glemail.com> wrote: > > On 7 April 2017 at 15:14, Thomas Gleixner <tglx@...utronix.de> wrote: > >> On Fri, 7 Apr 2017, Mathias Krause wrote: > > Fair enough. However, placing a BUG_ON(!(read_cr0() & X86_CR0_WP)) > > somewhere sensible should make those "leaks" visible fast -- and their > > exploitation impossible, i.e. fail hard. > > The leaks surely exist and now we'll just add an exploitable BUG. can you please share those leaks that 'surely exist' and CC oss-security while at it? > I think we're approaching this all wrong, actually. The fact that x86 > has this CR0.WP thing is arguably a historical accident, and the fact > that PaX uses it doesn't mean that PaX is doing it the best way for > upstream Linux. > > Why don't we start at the other end and do a generic non-arch-specific > implementation: set up an mm_struct that contains an RW alias of the > relevant parts of rodata and use use_mm to access it. (That is, > get_fs() to back up the old fs, set_fs(USER_DS), > use_mm(&rare_write_mm), do the write using copy_to_user, undo > everything.) > > Then someone who cares about performance can benchmark the CR0.WP > approach against it and try to argue that it's a good idea. This > benchmark should wait until I'm done with my PCID work, because PCID > is going to make use_mm() a whole heck of a lot faster. in my measurements switching PCID is hovers around 230 cycles for snb-ivb and 200-220 for hsw-skl whereas cr0 writes are around 230-240 cycles. there's of course a whole lot more impact for switching address spaces so it'll never be fast enough to beat cr0.wp.
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