netdev - Re: Alignment in BPF verifier

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Date:   Tue, 23 May 2017 18:43:48 +0100
From:   Edward Cree <ecree@...arflare.com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>
CC:     <ast@...com>, <alexei.starovoitov@...il.com>,
        <netdev@...r.kernel.org>
Subject: Re: Alignment in BPF verifier

Another issue: it looks like the min/max_value handling for subtraction is
 bogus.  In adjust_reg_min_max_vals() we have
    if (dst_reg->min_value != BPF_REGISTER_MIN_RANGE)
        dst_reg->min_value -= min_val;
    if (dst_reg->max_value != BPF_REGISTER_MAX_RANGE)
        dst_reg->max_value -= max_val;
 where min_val and max_val refer to the src_reg.
But surely they should be used the other way round; if (say) 2 <= R1 <= 6
 and 1 <= R2 <= 4, then this will claim 1 <= (R1 - R2) <= 2, whereas really
 (R1 - R2) could be anything from -2 to 5.
This also means that the code just above the switch,
    if (min_val == BPF_REGISTER_MIN_RANGE)
        dst_reg->min_value = BPF_REGISTER_MIN_RANGE;
    if (max_val == BPF_REGISTER_MAX_RANGE)
        dst_reg->max_value = BPF_REGISTER_MAX_RANGE;
 is wrong, since e.g. subtracting MAX_RANGE needs to blow our min_value,
 not our max_value.

-Ed