netdev - Re: [PATCH v2 bpf] bpf: prevent out-of-bounds speculation

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Date:   Fri, 5 Jan 2018 11:38:42 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Alexei Starovoitov <ast@...com>,
        "David S . Miller" <davem@...emloft.net>
Cc:     Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org
Subject: Re: [PATCH v2 bpf] bpf: prevent out-of-bounds speculation

On 01/04/2018 09:45 PM, 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.
> 
> 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.
> 
> If prog_array map is created by unpriv user replace
>   bpf_tail_call(ctx, map, index);
> with
>   if (index >= max_entries) {
>     index &= map->index_mask;
>     bpf_tail_call(ctx, map, index);
>   }
> (along with roundup to power 2) to prevent out-of-bounds speculation.
> There is secondary redundant 'if (index >= max_entries)' in the interpreter
> and in all JITs, but they can be optimized later if necessary.
> 
> Other array-like maps (cpumap, devmap, sockmap, perf_event_array, cgroup_array)
> cannot be used by unpriv, so no changes there.
> 
> That fixes bpf side of "Variant 1: bounds check bypass (CVE-2017-5753)" on
> all architectures with and without JIT.
> 
> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
> ---

LGTM, I'll drop it on my test systems and start running with it.
Although I don't have any Variant 1 code to test, but seems that
is being covered by others.

Thanks!

Acked-by: John Fastabend <john.fastabend@...il.com>