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Message-ID: <20170713091259.GA26194@leverpostej> Date: Thu, 13 Jul 2017 10:12:59 +0100 From: Mark Rutland <mark.rutland@....com> To: Ard Biesheuvel <ard.biesheuvel@...aro.org> Cc: linux-arm-kernel@...ts.infradead.org, kernel-hardening@...ts.openwall.com, labbott@...oraproject.org, will.deacon@....com, dave.martin@....com, catalin.marinas@....com Subject: Re: Re: [RFC PATCH 10/10] arm64: kernel: add support for virtually mapped stacks On Wed, Jul 12, 2017 at 11:59:38PM +0100, Mark Rutland wrote: > On Wed, Jul 12, 2017 at 03:44:23PM +0100, Ard Biesheuvel wrote: > > diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S > > index 2ba3185b1c78..4c3e82d6e2f2 100644 > > --- a/arch/arm64/kernel/entry.S > > +++ b/arch/arm64/kernel/entry.S > > @@ -392,6 +392,20 @@ ENDPROC(el1_error_invalid) > > */ > > .align 6 > > el1_sync: > > +#ifdef CONFIG_VMAP_STACK > > + /* > > + * When taking an exception from EL1, we need to check whether it is > > + * caused by a faulting out-of-bounds access to the virtually mapped > > + * stack before we can attempt to preserve the interrupted context. > > + */ > > + msr tpidrro_el0, x0 // stash x0 > > + mrs x0, far_el1 // get faulting address > > + tbnz x0, #63, .Lcheck_stack_ovf // check if not user address > > One thing I don't like about this is that we're reading FAR_EL1 even > though we don't know whether it's valid. It could contain junk, and we > could spuriously jump into the slow path. That's liable to make > performance erratic. Once I sent this, I realised what I said wasn't quite right. It's true that FAR_EL1 is UNKNOWN in some cases, but today those are all fatal (e.g. if we take an UNDEFINED exception due to a bad instruction at EL1). So my performance concern was spurious; sorry for the noise. However, so as to not mis-report those fatal cases, we either need to inspect the SP or ESR_EL1 to determine that FAR_EL1 is valid and/or this was a stack-overflow If we follow the approach of this series, we can move those checks into the check_stack_ovf slow path. Thanks, Mark.
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