Date: Mon, 30 Nov 2015 22:20:32 -0500 (EST) From: Nicolas Pitre <nicolas.pitre@...aro.org> To: Laura Abbott <labbott@...hat.com> cc: Kees Cook <keescook@...omium.org>, Russell King <linux@....linux.org.uk>, Ard Biesheuvel <ard.biesheuvel@...aro.org>, Will Deacon <will.deacon@....com>, Arnd Bergmann <arnd@...db.de>, Catalin Marinas <catalin.marinas@....com>, "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, LKML <linux-kernel@...r.kernel.org>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com> Subject: Re: [PATCH] ARM: mm: flip priority of CONFIG_DEBUG_RODATA On Mon, 30 Nov 2015, Laura Abbott wrote: > On 11/30/2015 05:08 PM, Kees Cook wrote: > > On Mon, Nov 30, 2015 at 5:03 PM, Laura Abbott <labbott@...hat.com> wrote: > > > On 11/30/2015 03:38 PM, Kees Cook wrote: > > > > > > > > Given the choice between making things NX or making things RO, we want > > > > RO first. As such, redefine CONFIG_DEBUG_RODATA to actually do the bulk > > > > > > > > > Can you give a citation for why? The thread that inspired it might be > > > a good link. > > > > This was inspired by my examining the existing architecture's > > implementations of CONFIG_DEBUG_RODATA after Ingo suggested it be made > > a common feature not a build-time config (or at least renamed): > > http://www.openwall.com/lists/kernel-hardening/2015/11/30/13 > > > > Thanks. I read the thread and I think it would be good to put a link > in the commit message to make it clearer why this is going in. A link is good, but a summary is even better. You may add both for best results. > > > > > index 41218867a9a6..b617084e9520 100644 > > > > --- a/arch/arm/mm/Kconfig > > > > +++ b/arch/arm/mm/Kconfig > > > > @@ -1039,24 +1039,26 @@ config ARCH_SUPPORTS_BIG_ENDIAN > > > > This option specifies the architecture can support big endian > > > > operation. > > > > > > > > -config ARM_KERNMEM_PERMS > > > > - bool "Restrict kernel memory permissions" > > > > +config DEBUG_RODATA > > > > + bool "Make kernel text and rodata read-only" > > > > depends on MMU > > > > + default y if CPU_V7 > > > > help > > > > - If this is set, kernel memory other than kernel text (and > > > > rodata) > > > > - will be made non-executable. 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), wasting memory. > > > > + If this is set, kernel memory (text, rodata, etc) will be made > > > > + read-only, and non-text kernel memory will be made > > > > non-executable. > > > > + 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. > > > > > > > > -config DEBUG_RODATA > > > > - bool "Make kernel text and rodata read-only" > > > > - depends on ARM_KERNMEM_PERMS > > > > +config DEBUG_ALIGN_RODATA > > > > + bool "Make rodata strictly non-executable" > > > > + depends on DEBUG_RODATA > > > > default y > > > > help > > > > - If this is set, kernel text and rodata will be made read-only. > > > > This > > > > - is to help catch accidental or malicious attempts to change > > > > the > > > > - kernel's executable code. Additionally splits rodata from > > > > kernel > > > > - text so it can be made explicitly non-executable. This creates > > > > - another section-size padded region, so it can waste more > > > > memory > > > > - space while gaining the read-only protections. > > > > + If this is set, rodata will be made explicitly non-executable. > > > > This > > > > + provides protection on the rare chance that attackers might > > > > find > > > > and > > > > + use ROP gadgets that exist in the rodata section. This adds an > > > > + additional section-aligned split of rodata from kernel text so > > > > it > > > > + can be made explicitly non-executable. This padding may waste > > > > memory > > > > + space to gain this additional protection. > > > > > > > > > I get that you want to make this match arm64 but it's really not intuitive > > > that > > > something with ALIGN_RODATA in the name is actually for setting NX. The > > > purpose > > > of ALIGN_RODATA was also slightly different on arm64 since the RO/NX will > > > still > > > be there, the difference is if the sections are present versus broken down > > > into > > > pages. > > > > Well, it seems to have the same effect: without the alignment, a > > portion of rodata may remain executable on arm64. Unless I > > misunderstand? > > > > No, on arm64 everything should always be NX, the difference is part of the NX > sections may be mapped as pages instead of sections so you take the TLB hit. > It's a trade off of memory vs TLB pressure instead of just security vs TLB. > > Thanks, > Laura > >
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