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Date: Mon, 6 Aug 2018 12:50:19 -0700
From: Kees Cook <keescook@...omium.org>
To: Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc: Robin Murphy <robin.murphy@....com>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>, Mark Rutland <mark.rutland@....com>,
Catalin Marinas <catalin.marinas@....com>, Will Deacon <will.deacon@....com>,
Christoffer Dall <christoffer.dall@....com>,
linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
Laura Abbott <labbott@...oraproject.org>, Julien Thierry <julien.thierry@....com>
Subject: Re: [RFC/PoC PATCH 0/3] arm64: basic ROP mitigation
On Mon, Aug 6, 2018 at 12:35 PM, Ard Biesheuvel
<ard.biesheuvel@...aro.org> wrote:
> On 6 August 2018 at 20:49, Kees Cook <keescook@...omium.org> wrote:
>> On Mon, Aug 6, 2018 at 10:45 AM, Robin Murphy <robin.murphy@....com> wrote:
>>> I guess what I'm getting at is that if the protection mechanism is "always
>>> return with SP outside TTBR1", there seems little point in going through the
>>> motions if SP in TTBR0 could still be valid and allow an attack to succeed
>>> anyway; this is basically just me working through a justification for saying
>>> the proposed scheme needs "depends on ARM64_PAN || ARM64_SW_TTBR0_PAN",
>>> making it that much uglier for v8.0 CPUs...
>>
>> I think anyone with v8.0 CPUs interested in this mitigation would also
>> very much want PAN emulation. If a "depends on" isn't desired, what
>> about "imply" in the Kconfig?
>>
>
> Yes, but actually, using bit #0 is maybe a better alternative in any
> case. You can never dereference SP with bit #0 set, regardless of
> whether the address points to user or kernel space, and my concern
> about reloading sp from x29 doesn't really make sense, given that x29
> is always assigned from sp right after pushing x29 and x30 in the
> function prologue, and sp only gets restored from x29 in the epilogue
> when there is a stack frame to begin with, in which case we add #1 to
> sp again before returning from the function.
Fair enough! :)
> The other code gets a lot cleaner as well.
>
> So for the return we'll have
>
> ldp x29, x30, [sp], #nn
>>>add sp, sp, #0x1
> ret
>
> and for the function call
>
> bl <foo>
>>>mov x30, sp
>>>bic sp, x30, #1
>
> The restore sequence in entry.s:96 (which has no spare registers) gets
> much simpler as well:
>
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -95,6 +95,15 @@ alternative_else_nop_endif
> */
> add sp, sp, x0 // sp' = sp + x0
> sub x0, sp, x0 // x0' = sp' - x0 = (sp + x0) - x0 = sp
> +#ifdef CONFIG_ARM64_ROP_SHIELD
> + tbnz x0, #0, 1f
> + .subsection 1
> +1: sub x0, x0, #1
> + sub sp, sp, #1
> + b 2f
> + .previous
> +2:
> +#endif
> tbnz x0, #THREAD_SHIFT, 0f
> sub x0, sp, x0 // x0'' = sp' - x0' = (sp + x0) - sp = x0
> sub sp, sp, x0 // sp'' = sp' - x0 = (sp + x0) - x0 = sp
I get slightly concerned about "add" vs "clear bit", but I don't see a
real way to chain a lot of "add"s to get to avoid the unaligned
access. Is "or" less efficient than "add"?
-Kees
--
Kees Cook
Pixel Security
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