netdev - Re: [PATCH bpf] bpf: prevent out-of-bounds speculation

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Date:   Mon, 8 Jan 2018 17:05:53 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Alexei Starovoitov <ast@...com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jann Horn <jannh@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Alan Cox <alan@...ux.intel.com>, netdev@...r.kernel.org,
        kernel-team@...com, will.deacon@....com
Subject: Re: [PATCH bpf] bpf: prevent out-of-bounds speculation

Hi Alexei,

On Thu, Jan 04, 2018 at 08:28:11PM -0800, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@...nel.org>
> 
> Under speculation, CPUs may mis-predict branches in bounds checks. Thus,
> memory accesses under a bounds check may be speculated even if the
> bounds check fails, providing a primitive for building a side channel.
> 
> To avoid leaking kernel data round up array-based maps and mask the index
> after bounds check, so speculated load with out of bounds index will load
> either valid value from the array or zero from the padded area.

Thanks for putting this together, this certainly looks neat.

I'm a little worried that in the presence of some CPU/compiler
optimisations, the masking may effectively be skipped under speculation.
So I'm not sure how robust this is going to be.

More on that below.

> To avoid duplicating map_lookup functions for root/unpriv always generate
> a sequence of bpf instructions equivalent to map_lookup function for
> array and array_of_maps map types when map was created by unpriv user.
> And unconditionally mask index for percpu_array, since it's fast enough,
> even when max_entries are not rounded to power of 2 for root user,
> since percpu_array doesn't have map_gen_lookup callback yet.

Is there a noticeable slowdown from the masking? Can't we always have
that in place?

> @@ -157,7 +175,7 @@ static void *percpu_array_map_lookup_elem(struct bpf_map *map, void *key)
>  	if (unlikely(index >= array->map.max_entries))
>  		return NULL;
>  
> -	return this_cpu_ptr(array->pptrs[index]);
> +	return this_cpu_ptr(array->pptrs[index & array->index_mask]);

As above, I think this isn't necessarily robust, as CPU/compiler
optimisations can break the dependency on the index_mask, allowing
speculation without a mask.

e.g. a compiler could re-write this as:

	if (array->index_mask != 0xffffffff)
		index &= array->index_mask;
	return this_cpu_ptr(array->pptrs[index]);

... which would allow an unmasked index to be used in speculated paths.

Similar cases could occur with some CPU implementations. For example, HW
value-prediction could result in the use of an all-ones mask under
speculation.

I think that we may need to be able to provide an arch-specific
pointer sanitization sequence (though we could certainly have masking as
the default).

I have a rough idea as to how that could be plumbed into the JIT. First
I need to verify the sequence I have in mind for arm/arm64 is
sufficient.

Thanks,
Mark.