From: Pablo Neira Ayuso <pablo@netfilter.org>
To: Florian Westphal <fw@strlen.de>
Cc: netfilter-devel@vger.kernel.org
Subject: Re: [PATCH nft 1/2] evaluate: shift immediate value when adjusting size for csum fixup
Date: Thu, 17 Aug 2017 12:00:56 +0200 [thread overview]
Message-ID: <20170817100056.GA6928@salvia> (raw)
In-Reply-To: <20170816170156.11594-1-fw@strlen.de>
On Wed, Aug 16, 2017 at 07:01:55PM +0200, Florian Westphal wrote:
> nft add rule .. ip ttl set 64
>
> erronously mangles ip protocol instead of ttl.
>
> Because the kernel can't deal with odd-sized data (ttl is one byte) when
> doing checksum fixups, so the write to 'ttl' is turned into
>
> [ payload load 2b @ network header + 8 => reg 1 ]
> [ bitwise reg 1 = (reg=1 & 0x000000ff ) ^ $new_value ]
> [ payload write reg 1 => 2b @ network header + 8 csum_type 1 csum_off 10 csum_flags 0x0 ]
>
> While doing so, we did fail to shift the imm value, i.e.
> we clear the wrong half of the u16 (protocol) instead of csum.
>
> The correct mask is 0xff00, and $new_value needs to be shifted
> so we leave the protocol value (which is next to ttl) alone.
>
> Fixes: f9069cefdf ("netlink: make checksum fixup work with odd-sized header fields")
> Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>
prev parent reply other threads:[~2017-08-17 10:01 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-08-16 17:01 [PATCH nft 1/2] evaluate: shift immediate value when adjusting size for csum fixup Florian Westphal
2017-08-16 17:01 ` [PATCH nft 2/2] tests: add test case for ttl/protocol set Florian Westphal
2017-08-17 10:01 ` Pablo Neira Ayuso
2017-08-17 10:00 ` Pablo Neira Ayuso [this message]
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