Date: Mon, 7 Aug 2017 14:47:08 -0700 From: Kees Cook <keescook@...omium.org> To: Ard Biesheuvel <ard.biesheuvel@...aro.org> Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Mark Rutland <mark.rutland@....com>, Catalin Marinas <catalin.marinas@....com>, James Morse <james.morse@....com>, Laura Abbott <labbott@...hat.com>, Andy Lutomirski <luto@...capital.net>, Matt Fleming <matt@...eblueprint.co.uk>, Will Deacon <will.deacon@....com>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org> Subject: Re: [PATCH] lkdtm: Test VMAP_STACK allocates leading/trailing guard pages On Mon, Aug 7, 2017 at 2:46 PM, Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote: > On 7 August 2017 at 22:44, Kees Cook <keescook@...omium.org> wrote: >> On Mon, Aug 7, 2017 at 2:27 PM, Ard Biesheuvel >> <ard.biesheuvel@...aro.org> wrote: >>> On 7 August 2017 at 21:39, Kees Cook <keescook@...omium.org> wrote: >>>> Two new tests STACK_GUARD_PAGE_LEADING and STACK_GUARD_PAGE_TRAILING >>>> attempt to read the byte before and after, respectively, of the current >>>> stack frame, which should fault under VMAP_STACK. >>>> >>>> Signed-off-by: Kees Cook <keescook@...omium.org> >>>> --- >>>> Do these tests both trip with the new arm64 VMAP_STACK code? >>> >>> Probably not. On arm64, the registers are stacked by software at >>> exception entry, and so we decrement the sp first by the size of the >>> register file, and if the resulting value overflows the stack, the >>> situation is handled as if the exception was caused by a faulting >>> stack access while it may be caused by something else in reality. >>> Since the act of handling the exception is guaranteed to overflow the >>> stack anyway, this does not really make a huge difference, and it >>> prevents the recursive fault from wiping the context that we need to >>> produce the diagnostics. >>> >>> This means an illegal access right above the stack will go undetected. >> >> I thought vmap entries provided guard pages around allocations? >> Shouldn't that catch it? >> > > Ah yes, so we will fault. We should probably double check whether we > will not misidentify the fault because of the subtraction we do first, > but that should be trivial to add. Okay, cool. I'd be curious to see what the lkdtm tests show for you. -Kees -- Kees Cook Pixel Security
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