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Message-ID: <534ACAB5.5030500@redhat.com>
Date: Sun, 13 Apr 2014 19:34:45 +0200
From: Daniel Borkmann <dborkman@...hat.com>
To: Mathias Krause <minipli@...glemail.com>
CC: "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
Patrick McHardy <kaber@...sh.net>,
Pablo Neira Ayuso <pablo@...filter.org>
Subject: Re: [PATCH net] filter: prevent nla extensions to peek beyond the
end of the message
On 04/13/2014 06:23 PM, Mathias Krause wrote:
> The BPF_S_ANC_NLATTR and BPF_S_ANC_NLATTR_NEST extensions fail to check
> for a minimal message length before testing the supplied offset to be
> within the bounds of the message. This allows the subtraction of the nla
> header to underflow and therefore -- as the data type is unsigned --
> allowing far to big offset and length values for the search of the
> netlink attribute.
>
> The remainder calculation for the BPF_S_ANC_NLATTR_NEST extension is
> also wrong. It has the minuend und subtrahend mixed up, therefore
> calculates a huge length value, allowing to overrun the end of the
> message while looking for the netlink attribute.
>
> The following three BPF snippets will trigger the bugs when attached to
> a UNIX datagram socket and parsing a message with length 1, 2 or 3.
>
> ,-[ PoC for missing size check in BPF_S_ANC_NLATTR ]--
> | ld #0x87654321
> | ldx #42
> | ld #nla
> | ret a
> `---
>
> ,-[ PoC for the same bug in BPF_S_ANC_NLATTR_NEST ]--
> | ld #0x87654321
> | ldx #42
> | ld #nlan
> | ret a
> `---
>
> ,-[ PoC for wrong remainder calculation in BPF_S_ANC_NLATTR_NEST ]--
> | ; (needs a fake netlink header at offset 0)
> | ld #0
> | ldx #42
> | ld #nlan
> | ret a
> `---
>
> Fix the first issue by ensuring the message length fulfills the minimal
> size constrains of a nla header. Fix the second bug by getting the math
> for the remainder calculation right.
>
> Fixes: 4738c1db15 ("[SKFILTER]: Add SKF_ADF_NLATTR instruction")
> Fixes: d214c7537b ("filter: add SKF_AD_NLATTR_NEST to look for nested..")
> Cc: Patrick McHardy <kaber@...sh.net>
> Cc: Pablo Neira Ayuso <pablo@...filter.org>
> Signed-off-by: Mathias Krause <minipli@...glemail.com>
Acked-by: Daniel Borkmann <dborkman@...hat.com>
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