Re: [PATCH net] filter: prevent nla extensions to peek beyond the end of the message

[Daniel Borkmann <dborkman@redhat.com>]

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@trash.net>
> Cc: Pablo Neira Ayuso <pablo@netfilter.org>
> Signed-off-by: Mathias Krause <minipli@googlemail.com>

Acked-by: Daniel Borkmann <dborkman@redhat.com>