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Date: Wed, 8 Aug 2018 15:02:26 -0700
From: Kees Cook <keescook@...omium.org>
To: Mark Brand <markbrand@...gle.com>
Cc: Ard Biesheuvel <ard.biesheuvel@...aro.org>, Catalin Marinas <catalin.marinas@....com>, 
	Christoffer Dall <christoffer.dall@....com>, Julien Thierry <julien.thierry@....com>, 
	Kernel Hardening <kernel-hardening@...ts.openwall.com>, 
	Laura Abbott <labbott@...oraproject.org>, Mark Rutland <mark.rutland@....com>, 
	Robin Murphy <robin.murphy@....com>, Will Deacon <will.deacon@....com>, 
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC/PoC PATCH 0/3] arm64: basic ROP mitigation

On Wed, Aug 8, 2018 at 9:09 AM, Mark Brand <markbrand@...gle.com> wrote:
> (1) chaining through the mempool_free function, I found this really
> quickly when searching for useful gadgets based off x0
>
> void mempool_free(void *element, mempool_t *pool)
> {
>   unsigned long flags;
>
>   if (unlikely(element == NULL))
>     return;
>
>   /* snip */
>   smp_rmb();
>
>   /* snip */
>   if (unlikely(pool->curr_nr < pool->min_nr)) {
>     spin_lock_irqsave(&pool->lock, flags);
>     if (likely(pool->curr_nr < pool->min_nr)) {
>       add_element(pool, element);
>       spin_unlock_irqrestore(&pool->lock, flags);
>       wake_up(&pool->wait);
>       return;
>     }
>     spin_unlock_irqrestore(&pool->lock, flags);
>   }
>
>   pool->free(element, pool->pool_data);
> }
>
> Since the callsites for this function usually load the arguments
> through some registers, and the function to call gets pulled out of
> one of those arguments, it's easy to get a couple of registers loaded
> here and then the chain continue.
>
> (2) loading complete register state using kernel_exit macro.
>
> Since the kernel_exit macro actually loads spsr_el1 and elr_el1 from
> registers, I think that you can let the eret return to anywhere in el1
> without dropping to el0, since the same handler is used for "exiting
> the kernel" when a hardware interrupt interrupts the kernel itself. I
> didn't fill out the necessary register values in the chain below,
> since I don't anyway have a device around to test this on right now.
>
> I'm not sure that you could really robustly protect this eret; I
> suppose that you could try and somehow validate the saved register
> state, but given that it would be happening on every exception return,
> I suspect it would be expensive.

This is a wonderful example, thanks! Yeah, Call-Oriented Programming?
:P mempool_free() is quite a nice gadget.

-Kees

-- 
Kees Cook
Pixel Security

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