Date: Thu, 16 Feb 2017 17:08:20 -0800 From: Kees Cook <keescook@...omium.org> To: Pavel Machek <pavel@....cz> Cc: Laura Abbott <labbott@...hat.com>, Mark Rutland <mark.rutland@....com>, "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>, Catalin Marinas <catalin.marinas@....com>, Heiko Carstens <heiko.carstens@...ibm.com>, "James E.J. Bottomley" <jejb@...isc-linux.org>, "H. Peter Anvin" <hpa@...or.com>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, Rob Herring <robh@...nel.org>, Jessica Yu <jeyu@...hat.com>, Jonathan Corbet <corbet@....net>, Helge Deller <deller@....de>, "x86@...nel.org" <x86@...nel.org>, Russell King <linux@...linux.org.uk>, Ingo Molnar <mingo@...hat.com>, Len Brown <len.brown@...el.com>, "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>, Will Deacon <will.deacon@....com>, Thomas Gleixner <tglx@...utronix.de>, "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, linux-parisc <linux-parisc@...r.kernel.org>, Linux PM list <linux-pm@...r.kernel.org>, "Rafael J. Wysocki" <rjw@...ysocki.net>, LKML <linux-kernel@...r.kernel.org>, Jason Wessel <jason.wessel@...driver.com>, Martin Schwidefsky <schwidefsky@...ibm.com>, Robin Murphy <robin.murphy@....com> Subject: Re: [PATCHv3 2/2] arch: Rename CONFIG_DEBUG_RODATA and CONFIG_DEBUG_MODULE_RONX On Thu, Feb 16, 2017 at 2:25 PM, Pavel Machek <pavel@....cz> wrote: > Hi! > >> >> -config DEBUG_RODATA >> +config STRICT_KERNEL_RWX >> bool "Make kernel text and rodata read-only" if ARCH_OPTIONAL_KERNEL_RWX >> depends on ARCH_HAS_STRICT_KERNEL_RWX >> default !ARCH_OPTIONAL_KERNEL_RWX || > > Debug features are expected to have runtime cost, so kconfig help is > silent about those. But there are runtime costs, right? It would be > nice to mention them in the help text... It depends on the architecture. The prior help text for arm said: The tradeoff is that each region is padded to section-size (1MiB) boundaries (because their permissions are different and splitting the 1M pages into 4K ones causes TLB performance problems), which can waste memory. parisc (somewhat inaccurately) said: This option may have a slight performance impact because a portion of the kernel code won't be covered by a TLB anymore. IIUC, arm64 does what parisc is hinting at: mappings at the end are broken down to PAGE_SIZE. On x86, IIUC, there's actually no change to TLB performance due to how the mappings are already set up. I'm not sure the best way to express this in the new help text. Do you have some suggestions on wording? Personally, I don't really think it's worth mentioning this in Kconfig help, which, in theory, is supposed to limit how technical it gets. And I think the performance impact is almost entirely negligible compared to the risks addressed. -Kees -- Kees Cook Pixel Security
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